branch: add flags and config to inherit tracking

It can be helpful when creating a new branch to use the existing
tracking configuration from the branch point. However, there is
currently not a method to automatically do so.

Teach git-{branch,checkout,switch} an "inherit" argument to the
"--track" option. When this is set, creating a new branch will cause the
tracking configuration to default to the configuration of the branch
point, if set.

For example, if branch "main" tracks "origin/main", and we run
`git checkout --track=inherit -b feature main`, then branch "feature"
will track "origin/main". Thus, `git status` will show us how far
ahead/behind we are from origin, and `git pull` will pull from origin.

This is particularly useful when creating branches across many
submodules, such as with `git submodule foreach ...` (or if running with
a patch such as [1], which we use at $job), as it avoids having to
manually set tracking info for each submodule.

Since we've added an argument to "--track", also add "--track=direct" as
another way to explicitly get the original "--track" behavior ("--track"
without an argument still works as well).

Finally, teach branch.autoSetupMerge a new "inherit" option. When this
is set, "--track=inherit" becomes the default behavior.

[1]: https://lore.kernel.org/git/20180927221603.148025-1-sbeller@google.com/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/branch.txt

@@ -7,7 +7,8 @@ branch.autoSetupMerge::
 	automatic setup is done; `true` -- automatic setup is done when the
 	starting point is a remote-tracking branch; `always` --
 	automatic setup is done when the starting point is either a
-	local branch or remote-tracking
+	local branch or remote-tracking branch; `inherit` -- if the starting point
+	has a tracking configuration, it is copied to the new
 	branch. This option defaults to true.
 
 branch.autoSetupRebase::

Documentation/git-branch.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 	[--points-at <object>] [--format=<format>]
 	[(-r | --remotes) | (-a | --all)]
 	[--list] [<pattern>...]
-'git branch' [--track | --no-track] [-f] <branchname> [<start-point>]
+'git branch' [--track [direct|inherit] | --no-track] [-f] <branchname> [<start-point>]
 'git branch' (--set-upstream-to=<upstream> | -u <upstream>) [<branchname>]
 'git branch' --unset-upstream [<branchname>]
 'git branch' (-m | -M) [<oldbranch>] <newbranch>
@@ -206,24 +206,34 @@ This option is only applicable in non-verbose mode.
 	Display the full sha1s in the output listing rather than abbreviating them.
 
 -t::
---track::
+--track [inherit|direct]::
 	When creating a new branch, set up `branch.<name>.remote` and
-	`branch.<name>.merge` configuration entries to mark the
-	start-point branch as "upstream" from the new branch. This
+	`branch.<name>.merge` configuration entries to set "upstream" tracking
+	configuration for the new branch. This
 	configuration will tell git to show the relationship between the
 	two branches in `git status` and `git branch -v`. Furthermore,
 	it directs `git pull` without arguments to pull from the
 	upstream when the new branch is checked out.
 +
-This behavior is the default when the start point is a remote-tracking branch.
+The exact upstream branch is chosen depending on the optional argument:
+`--track` or `--track direct` means to use the start-point branch itself as the
+upstream; `--track inherit` means to copy the upstream configuration of the
+start-point branch.
++
+`--track direct` is the default when the start point is a remote-tracking branch.
 Set the branch.autoSetupMerge configuration variable to `false` if you
 want `git switch`, `git checkout` and `git branch` to always behave as if `--no-track`
 were given. Set it to `always` if you want this behavior when the
-start-point is either a local or remote-tracking branch.
+start-point is either a local or remote-tracking branch. Set it to
+`inherit` if you want to copy the tracking configuration from the
+branch point.
++
+See linkgit:git-pull[1] and linkgit:git-config[1] for additional discussion on
+how the `branch.<name>.remote` and `branch.<name>.merge` options are used.
 
 --no-track::
 	Do not set up "upstream" configuration, even if the
-	branch.autoSetupMerge configuration variable is true.
+	branch.autoSetupMerge configuration variable is set.
 
 --set-upstream::
 	As this option had confusing syntax, it is no longer supported.

Documentation/git-checkout.txt

@@ -156,7 +156,7 @@ of it").
 	linkgit:git-branch[1] for details.
 
 -t::
---track::
+--track [direct|inherit]::
 	When creating a new branch, set up "upstream" configuration. See
 	"--track" in linkgit:git-branch[1] for details.
 +

Documentation/git-switch.txt

@@ -152,7 +152,7 @@ should result in deletion of the path).
 	attached to a terminal, regardless of `--quiet`.
 
 -t::
---track::
+--track [direct|inherit]::
 	When creating a new branch, set up "upstream" configuration.
 	`-c` is implied. See `--track` in linkgit:git-branch[1] for
 	details.