merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/Documentation/git-submodule.txt

@@ -16,6 +16,7 @@ SYNOPSIS
 'git submodule' [--quiet] deinit [-f|--force] (--all|[--] <path>...)
 'git submodule' [--quiet] update [<options>] [--] [<path>...]
 'git submodule' [--quiet] set-branch [<options>] [--] <path>
+'git submodule' [--quiet] set-url [--] <path> <newurl>
 'git submodule' [--quiet] summary [<options>] [--] [<path>...]
 'git submodule' [--quiet] foreach [--recursive] <command>
 'git submodule' [--quiet] sync [--recursive] [--] [<path>...]
@@ -80,6 +81,9 @@ status [--cached] [--recursive] [--] [<path>...]::
 	does not match the SHA-1 found in the index of the containing
 	repository and `U` if the submodule has merge conflicts.
 +
+If `--cached` is specified, this command will instead print the SHA-1
+recorded in the superproject for each submodule.
++
 If `--recursive` is specified, this command will recurse into nested
 submodules, and show their status as well.
 +
@@ -129,11 +133,12 @@ If you really want to remove a submodule from the repository and commit
 that use linkgit:git-rm[1] instead. See linkgit:gitsubmodules[7] for removal
 options.
 
-update [--init] [--remote] [-N|--no-fetch] [--[no-]recommend-shallow] [-f|--force] [--checkout|--rebase|--merge] [--reference <repository>] [--depth <depth>] [--recursive] [--jobs <n>] [--] [<path>...]::
+update [--init] [--remote] [-N|--no-fetch] [--[no-]recommend-shallow] [-f|--force] [--checkout|--rebase|--merge] [--reference <repository>] [--depth <depth>] [--recursive] [--jobs <n>] [--[no-]single-branch] [--] [<path>...]::
 +
 --
 Update the registered submodules to match what the superproject
-expects by cloning missing submodules and updating the working tree of
+expects by cloning missing submodules, fetching missing commits
+in submodules and updating the working tree of
 the submodules. The "updating" can be done in several ways depending
 on command line options and the value of `submodule.<name>.update`
 configuration variable. The command line option takes precedence over
@@ -173,11 +178,17 @@ submodule with the `--init` option.
 If `--recursive` is specified, this command will recurse into the
 registered submodules, and update any nested submodules within.
 --
-set-branch ((-d|--default)|(-b|--branch <branch>)) [--] <path>::
+set-branch (-b|--branch) <branch> [--] <path>::
+set-branch (-d|--default) [--] <path>::
 	Sets the default remote tracking branch for the submodule. The
 	`--branch` option allows the remote branch to be specified. The
 	`--default` option removes the submodule.<name>.branch configuration
-	key, which causes the tracking branch to default to 'master'.
+	key, which causes the tracking branch to default to the remote 'HEAD'.
+
+set-url [--] <path> <newurl>::
+	Sets the URL of the specified submodule to <newurl>. Then, it will
+	automatically synchronize the submodule's new remote URL
+	configuration.
 
 summary [--cached|--files] [(-n|--summary-limit) <n>] [commit] [--] [<path>...]::
 	Show commit summary between the given commit (defaults to HEAD) and
@@ -218,7 +229,7 @@ As an example, the command below will show the path and currently
 checked out commit for each submodule:
 +
 --------------
-git submodule foreach 'echo $path `git rev-parse HEAD`'
+git submodule foreach 'echo $sm_path `git rev-parse HEAD`'
 --------------
 
 sync [--recursive] [--] [<path>...]::
@@ -237,7 +248,7 @@ registered submodules, and sync any nested submodules within.
 
 absorbgitdirs::
 	If a git directory of a submodule is inside the submodule,
-	move the git directory of the submodule into its superprojects
+	move the git directory of the submodule into its superproject's
 	`$GIT_DIR/modules` path and then connect the git directory and
 	its working directory by setting the `core.worktree` and adding
 	a .git file pointing to the git directory embedded in the
@@ -273,7 +284,7 @@ OPTIONS
 	`.gitmodules` for `update --remote`.  A special value of `.` is used to
 	indicate that the name of the branch in the submodule should be the
 	same name as the current branch in the current repository.  If the
-	option is not specified, it defaults to 'master'.
+	option is not specified, it defaults to the remote 'HEAD'.
 
 -f::
 --force::
@@ -311,10 +322,10 @@ OPTIONS
 	the superproject's recorded SHA-1 to update the submodule, use the
 	status of the submodule's remote-tracking branch.  The remote used
 	is branch's remote (`branch.<name>.remote`), defaulting to `origin`.
-	The remote branch used defaults to `master`, but the branch name may
-	be overridden by setting the `submodule.<name>.branch` option in
-	either `.gitmodules` or `.git/config` (with `.git/config` taking
-	precedence).
+	The remote branch used defaults to the remote `HEAD`, but the branch
+	name may be overridden by setting the `submodule.<name>.branch`
+	option in either `.gitmodules` or `.git/config` (with `.git/config`
+	taking precedence).
 +
 This works for any of the supported update procedures (`--checkout`,
 `--rebase`, etc.).  The only change is the source of the target SHA-1.
@@ -419,6 +430,10 @@ options carefully.
 	Clone new submodules in parallel with as many jobs.
 	Defaults to the `submodule.fetchJobs` option.
 
+--[no-]single-branch::
+	This option is only valid for the update command.
+	Clone only one branch during update: HEAD or one specified by --branch.
+
 <path>...::
 	Paths to submodule(s). When specified this will restrict the command
 	to only operate on the submodules found at the specified paths.