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/fetch-options.txt

@@ -61,9 +61,16 @@ this option multiple times, one for each matching ref name.
 See also the `fetch.negotiationAlgorithm` configuration variable
 documented in linkgit:git-config[1].
 
-ifndef::git-pull[]
 --dry-run::
 	Show what would be done, without making any changes.
+
+ifndef::git-pull[]
+--[no-]write-fetch-head::
+	Write the list of remote refs fetched in the `FETCH_HEAD`
+	file directly under `$GIT_DIR`.  This is the default.
+	Passing `--no-write-fetch-head` from the command line tells
+	Git not to write the file.  Under `--dry-run` option, the
+	file is never written.
 endif::git-pull[]
 
 -f::
@@ -88,9 +95,16 @@ ifndef::git-pull[]
 	Allow several <repository> and <group> arguments to be
 	specified. No <refspec>s may be specified.
 
+--[no-]auto-maintenance::
 --[no-]auto-gc::
-	Run `git gc --auto` at the end to perform garbage collection
-	if needed. This is enabled by default.
+	Run `git maintenance run --auto` at the end to perform automatic
+	repository maintenance if needed. (`--[no-]auto-gc` is a synonym.)
+	This is enabled by default.
+
+--[no-]write-commit-graph::
+	Write a commit-graph after fetching. This overrides the config
+	setting `fetch.writeCommitGraph`.
+endif::git-pull[]
 
 -p::
 --prune::
@@ -103,6 +117,7 @@ ifndef::git-pull[]
 	was cloned with the --mirror option), then they are also
 	subject to pruning. Supplying `--prune-tags` is a shorthand for
 	providing the tag refspec.
+ifndef::git-pull[]
 +
 See the PRUNING section below for more details.
 
@@ -129,13 +144,15 @@ endif::git-pull[]
 	behavior for a remote may be specified with the remote.<name>.tagOpt
 	setting. See linkgit:git-config[1].
 
-ifndef::git-pull[]
 --refmap=<refspec>::
 	When fetching refs listed on the command line, use the
 	specified refspec (can be given more than once) to map the
 	refs to remote-tracking branches, instead of the values of
 	`remote.*.fetch` configuration variables for the remote
-	repository.  See section on "Configured Remote-tracking
+	repository.  Providing an empty `<refspec>` to the
+	`--refmap` option causes Git to ignore the configured
+	refspecs and rely entirely on the refspecs supplied as
+	command-line arguments. See section on "Configured Remote-tracking
 	Branches" for details.
 
 -t::
@@ -147,6 +164,7 @@ ifndef::git-pull[]
 	is used (though tags may be pruned anyway if they are also the
 	destination of an explicit refspec; see `--prune`).
 
+ifndef::git-pull[]
 --recurse-submodules[=yes|on-demand|no]::
 	This option controls if and under what conditions new commits of
 	populated submodules should be fetched too. It can be used as a
@@ -156,19 +174,36 @@ ifndef::git-pull[]
 	value. Use 'on-demand' to only recurse into a populated submodule
 	when the superproject retrieves a commit that updates the submodule's
 	reference to a commit that isn't already in the local submodule
-	clone.
+	clone. By default, 'on-demand' is used, unless
+	`fetch.recurseSubmodules` is set (see linkgit:git-config[1]).
+endif::git-pull[]
 
 -j::
 --jobs=<n>::
-	Number of parallel children to be used for fetching submodules.
-	Each will fetch from different submodules, such that fetching many
-	submodules will be faster. By default submodules will be fetched
-	one at a time.
+	Number of parallel children to be used for all forms of fetching.
++
+If the `--multiple` option was specified, the different remotes will be fetched
+in parallel. If multiple submodules are fetched, they will be fetched in
+parallel. To control them independently, use the config settings
+`fetch.parallel` and `submodule.fetchJobs` (see linkgit:git-config[1]).
++
+Typically, parallel recursive and multi-remote fetches will be faster. By
+default fetches are performed sequentially, not in parallel.
 
+ifndef::git-pull[]
 --no-recurse-submodules::
 	Disable recursive fetching of submodules (this has the same effect as
 	using the `--recurse-submodules=no` option).
+endif::git-pull[]
 
+--set-upstream::
+	If the remote is fetched successfully, add upstream
+	(tracking) reference, used by argument-less
+	linkgit:git-pull[1] and other commands. For more information,
+	see `branch.<name>.merge` and `branch.<name>.remote` in
+	linkgit:git-config[1].
+
+ifndef::git-pull[]
 --submodule-prefix=<path>::
 	Prepend <path> to paths printed in informative messages
 	such as "Fetching submodule foo".  This option is used
@@ -181,7 +216,6 @@ ifndef::git-pull[]
 	recursion (such as settings in linkgit:gitmodules[5] and
 	linkgit:git-config[1]) override this option, as does
 	specifying --[no-]recurse-submodules directly.
-endif::git-pull[]
 
 -u::
 --update-head-ok::
@@ -191,6 +225,7 @@ endif::git-pull[]
 	to communicate with 'git fetch', and unless you are
 	implementing your own Porcelain you are not supposed to
 	use it.
+endif::git-pull[]
 
 --upload-pack <upload-pack>::
 	When given, and the repository to fetch from is handled