doc: consolidate extensions in git-config documentation

The `technical/repository-version.txt` document originally served as the
master list for extensions, requiring that any new extensions be defined
there. However, the `config/extensions.txt` file was introduced later
and has since become the de facto location for describing extensions,
with several extensions listed there but missing from
`repository-version.txt`.

This consolidates all extension definitions into `config/extensions.txt`,
making it the authoritative source for extensions. The references in
`repository-version.txt` are updated to point to `config/extensions.txt`,
and cross-references to related documentation such as
`gitrepository-layout[5]` and `git-config[1]` are added.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Caleb White <cdwhite3@pm.me>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/config/core.txt

@@ -366,7 +366,7 @@ default in a bare repository.
 
 core.repositoryFormatVersion::
 	Internal variable identifying the repository format and layout
-	version.
+	version. See linkgit:gitrepository-layout[5].
 
 core.sharedRepository::
 	When 'group' (or 'true'), the repository is made shareable between

Documentation/config/extensions.txt

@@ -1,17 +1,13 @@
-extensions.objectFormat::
-	Specify the hash algorithm to use.  The acceptable values are `sha1` and
-	`sha256`.  If not specified, `sha1` is assumed.  It is an error to specify
-	this key unless `core.repositoryFormatVersion` is 1.
+extensions.*::
+	Unless otherwise stated, is an error to specify an extension if
+	`core.repositoryFormatVersion` is not `1`. See
+	linkgit:gitrepository-layout[5].
 +
-Note that this setting should only be set by linkgit:git-init[1] or
-linkgit:git-clone[1].  Trying to change it after initialization will not
-work and will produce hard-to-diagnose issues.
-
-extensions.compatObjectFormat::
-
+--
+compatObjectFormat::
 	Specify a compatibility hash algorithm to use.  The acceptable values
 	are `sha1` and `sha256`.  The value specified must be different from the
-	value of extensions.objectFormat.  This allows client level
+	value of `extensions.objectFormat`.  This allows client level
 	interoperability between git repositories whose objectFormat matches
 	this compatObjectFormat.  In particular when fully implemented the
 	pushes and pulls from a repository in whose objectFormat matches
@@ -19,18 +15,55 @@ extensions.compatObjectFormat::
 	compatObjectFormat in addition to oids encoded with objectFormat to
 	locally specify objects.
 
-extensions.refStorage::
+noop::
+	This extension does not change git's behavior at all. It is useful only
+	for testing format-1 compatibility.
++
+For historical reasons, this extension is respected regardless of the
+`core.repositoryFormatVersion` setting.
+
+noop-v1::
+	This extension does not change git's behavior at all. It is useful only
+	for testing format-1 compatibility.
+
+objectFormat::
+	Specify the hash algorithm to use.  The acceptable values are `sha1` and
+	`sha256`.  If not specified, `sha1` is assumed.
++
+Note that this setting should only be set by linkgit:git-init[1] or
+linkgit:git-clone[1].  Trying to change it after initialization will not
+work and will produce hard-to-diagnose issues.
+
+partialClone::
+	When enabled, indicates that the repo was created with a partial clone
+	(or later performed a partial fetch) and that the remote may have
+	omitted sending certain unwanted objects.  Such a remote is called a
+	"promisor remote" and it promises that all such omitted objects can
+	be fetched from it in the future.
++
+The value of this key is the name of the promisor remote.
++
+For historical reasons, this extension is respected regardless of the
+`core.repositoryFormatVersion` setting.
+
+preciousObjects::
+	If enabled, indicates that objects in the repository MUST NOT be deleted
+	(e.g., by `git-prune` or `git repack -d`).
++
+For historical reasons, this extension is respected regardless of the
+`core.repositoryFormatVersion` setting.
+
+refStorage::
 	Specify the ref storage format to use. The acceptable values are:
 +
 include::../ref-storage-format.txt[]
-+
-It is an error to specify this key unless `core.repositoryFormatVersion` is 1.
+
 +
 Note that this setting should only be set by linkgit:git-init[1] or
 linkgit:git-clone[1]. Trying to change it after initialization will not
 work and will produce hard-to-diagnose issues.
 
-extensions.worktreeConfig::
+worktreeConfig::
 	If enabled, then worktrees will load config settings from the
 	`$GIT_DIR/config.worktree` file in addition to the
 	`$GIT_COMMON_DIR/config` file. Note that `$GIT_COMMON_DIR` and
@@ -40,7 +73,7 @@ extensions.worktreeConfig::
 	`config.worktree` file will override settings from any other
 	config files.
 +
-When enabling `extensions.worktreeConfig`, you must be careful to move
+When enabling this extension, you must be careful to move
 certain values from the common config file to the main working tree's
 `config.worktree` file, if present:
 +
@@ -48,15 +81,17 @@ certain values from the common config file to the main working tree's
   `$GIT_COMMON_DIR/config.worktree`.
 * If `core.bare` is true, then it must be moved from `$GIT_COMMON_DIR/config`
   to `$GIT_COMMON_DIR/config.worktree`.
+
 +
 It may also be beneficial to adjust the locations of `core.sparseCheckout`
 and `core.sparseCheckoutCone` depending on your desire for customizable
 sparse-checkout settings for each worktree. By default, the `git
-sparse-checkout` builtin enables `extensions.worktreeConfig`, assigns
+sparse-checkout` builtin enables this extension, assigns
 these config values on a per-worktree basis, and uses the
 `$GIT_DIR/info/sparse-checkout` file to specify the sparsity for each
 worktree independently. See linkgit:git-sparse-checkout[1] for more
 details.
 +
-For historical reasons, `extensions.worktreeConfig` is respected
-regardless of the `core.repositoryFormatVersion` setting.
+For historical reasons, this extension is respected regardless of the
+`core.repositoryFormatVersion` setting.
+--