scalar: include in standard Git build & installation

Move 'scalar' out of 'contrib/' and into the root of the Git tree. The goal
of this change is to build 'scalar' as part of the standard Git build &
install processes.

This patch includes both the physical move of Scalar's files out of
'contrib/' ('scalar.c', 'scalar.txt', and 't9xxx-scalar.sh'), and the
changes to the build definitions in 'Makefile' and 'CMakelists.txt' to
accommodate the new program.

At a high level, Scalar is built so that:
- there is a 'scalar-objs' target (similar to those created in 029bac01a8
  (Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets,
  2021-02-23)) for debugging purposes.
- it appears in the root of the install directory (rather than the
  gitexecdir).
- it is included in the 'bin-wrappers/' directory for use in tests.
- it receives a platform-specific executable suffix (e.g., '.exe'), if
  applicable.
- 'scalar.txt' is installed as 'man1' documentation.
- the 'clean' target removes the 'scalar' executable.

Additionally, update the root level '.gitignore' file to ignore the Scalar
executable.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -21,6 +21,7 @@ MAN1_TXT += $(filter-out \
 MAN1_TXT += git.txt
 MAN1_TXT += gitk.txt
 MAN1_TXT += gitweb.txt
+MAN1_TXT += scalar.txt
 
 # man5 / man7 guides (note: new guides should also be added to command-list.txt)
 MAN5_TXT += gitattributes.txt

Documentation/scalar.txt

@@ -0,0 +1,166 @@
+scalar(1)
+=========
+
+NAME
+----
+scalar - A tool for managing large Git repositories
+
+SYNOPSIS
+--------
+[verse]
+scalar clone [--single-branch] [--branch <main-branch>] [--full-clone] <url> [<enlistment>]
+scalar list
+scalar register [<enlistment>]
+scalar unregister [<enlistment>]
+scalar run ( all | config | commit-graph | fetch | loose-objects | pack-files ) [<enlistment>]
+scalar reconfigure [ --all | <enlistment> ]
+scalar diagnose [<enlistment>]
+scalar delete <enlistment>
+
+DESCRIPTION
+-----------
+
+Scalar is a repository management tool that optimizes Git for use in large
+repositories. Scalar improves performance by configuring advanced Git settings,
+maintaining repositories in the background, and helping to reduce data sent
+across the network.
+
+An important Scalar concept is the enlistment: this is the top-level directory
+of the project. It usually contains the subdirectory `src/` which is a Git
+worktree. This encourages the separation between tracked files (inside `src/`)
+and untracked files, such as build artifacts (outside `src/`). When registering
+an existing Git worktree with Scalar whose name is not `src`, the enlistment
+will be identical to the worktree.
+
+The `scalar` command implements various subcommands, and different options
+depending on the subcommand. With the exception of `clone`, `list` and
+`reconfigure --all`, all subcommands expect to be run in an enlistment.
+
+The following options can be specified _before_ the subcommand:
+
+-C <directory>::
+	Before running the subcommand, change the working directory. This
+	option imitates the same option of linkgit:git[1].
+
+-c <key>=<value>::
+	For the duration of running the specified subcommand, configure this
+	setting. This option imitates the same option of linkgit:git[1].
+
+COMMANDS
+--------
+
+Clone
+~~~~~
+
+clone [<options>] <url> [<enlistment>]::
+	Clones the specified repository, similar to linkgit:git-clone[1]. By
+	default, only commit and tree objects are cloned. Once finished, the
+	worktree is located at `<enlistment>/src`.
++
+The sparse-checkout feature is enabled (except when run with `--full-clone`)
+and the only files present are those in the top-level directory. Use
+`git sparse-checkout set` to expand the set of directories you want to see,
+or `git sparse-checkout disable` to expand to all files (see
+linkgit:git-sparse-checkout[1] for more details). You can explore the
+subdirectories outside your sparse-checkout by using `git ls-tree
+HEAD[:<directory>]`.
+
+-b <name>::
+--branch <name>::
+	Instead of checking out the branch pointed to by the cloned
+	repository's HEAD, check out the `<name>` branch instead.
+
+--[no-]single-branch::
+	Clone only the history leading to the tip of a single branch, either
+	specified by the `--branch` option or the primary branch remote's
+	`HEAD` points at.
++
+Further fetches into the resulting repository will only update the
+remote-tracking branch for the branch this option was used for the initial
+cloning. If the HEAD at the remote did not point at any branch when
+`--single-branch` clone was made, no remote-tracking branch is created.
+
+--[no-]full-clone::
+	A sparse-checkout is initialized by default. This behavior can be
+	turned off via `--full-clone`.
+
+List
+~~~~
+
+list::
+	List enlistments that are currently registered by Scalar. This
+	subcommand does not need to be run inside an enlistment.
+
+Register
+~~~~~~~~
+
+register [<enlistment>]::
+	Adds the enlistment's repository to the list of registered repositories
+	and starts background maintenance. If `<enlistment>` is not provided,
+	then the enlistment associated with the current working directory is
+	registered.
++
+Note: when this subcommand is called in a worktree that is called `src/`, its
+parent directory is considered to be the Scalar enlistment. If the worktree is
+_not_ called `src/`, it itself will be considered to be the Scalar enlistment.
+
+Unregister
+~~~~~~~~~~
+
+unregister [<enlistment>]::
+	Remove the specified repository from the list of repositories
+	registered with Scalar and stop the scheduled background maintenance.
+
+Run
+~~~
+
+scalar run ( all | config | commit-graph | fetch | loose-objects | pack-files ) [<enlistment>]::
+	Run the given maintenance task (or all tasks, if `all` was specified).
+	Except for `all` and `config`, this subcommand simply hands off to
+	linkgit:git-maintenance[1] (mapping `fetch` to `prefetch` and
+	`pack-files` to `incremental-repack`).
++
+These tasks are run automatically as part of the scheduled maintenance,
+as soon as the repository is registered with Scalar. It should therefore
+not be necessary to run this subcommand manually.
++
+The `config` task is specific to Scalar and configures all those
+opinionated default settings that make Git work more efficiently with
+large repositories. As this task is run as part of `scalar clone`
+automatically, explicit invocations of this task are rarely needed.
+
+Reconfigure
+~~~~~~~~~~~
+
+After a Scalar upgrade, or when the configuration of a Scalar enlistment
+was somehow corrupted or changed by mistake, this subcommand allows to
+reconfigure the enlistment.
+
+With the `--all` option, all enlistments currently registered with Scalar
+will be reconfigured. Use this option after each Scalar upgrade.
+
+Diagnose
+~~~~~~~~
+
+diagnose [<enlistment>]::
+    When reporting issues with Scalar, it is often helpful to provide the
+    information gathered by this command, including logs and certain
+    statistics describing the data shape of the current enlistment.
++
+The output of this command is a `.zip` file that is written into
+a directory adjacent to the worktree in the `src` directory.
+
+Delete
+~~~~~~
+
+delete <enlistment>::
+	This subcommand lets you delete an existing Scalar enlistment from your
+	local file system, unregistering the repository.
+
+SEE ALSO
+--------
+linkgit:git-clone[1], linkgit:git-maintenance[1].
+
+GIT
+---
+Part of the linkgit:git[1] suite