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-pull.txt

@@ -85,8 +85,9 @@ OPTIONS
 	Pass --verbose to git-fetch and git-merge.
 
 --[no-]recurse-submodules[=yes|on-demand|no]::
-	This option controls if new commits of all populated submodules should
-	be fetched and updated, too (see linkgit:git-config[1] and
+	This option controls if new commits of populated submodules should
+	be fetched, and if the working trees of active submodules should be
+	updated, too (see linkgit:git-fetch[1], linkgit:git-config[1] and
 	linkgit:gitmodules[5]).
 +
 If the checkout is done via rebase, local submodule commits are rebased as well.
@@ -133,15 +134,6 @@ unless you have read linkgit:git-rebase[1] carefully.
 --no-rebase::
 	Override earlier --rebase.
 
---autostash::
---no-autostash::
-	Before starting rebase, stash local modifications away (see
-	linkgit:git-stash[1]) if needed, and apply the stash entry when
-	done. `--no-autostash` is useful to override the `rebase.autoStash`
-	configuration variable (see linkgit:git-config[1]).
-+
-This option is only valid when "--rebase" is used.
-
 Options related to fetching
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -229,9 +221,9 @@ branch.<name>.merge options; see linkgit:git-config[1] for details.
 $ git pull origin next
 ------------------------------------------------
 +
-This leaves a copy of `next` temporarily in FETCH_HEAD, but
-does not update any remote-tracking branches. Using remote-tracking
-branches, the same can be done by invoking fetch and merge:
+This leaves a copy of `next` temporarily in FETCH_HEAD, and
+updates the remote-tracking branch `origin/next`.
+The same can be done by invoking fetch and merge:
 +
 ------------------------------------------------
 $ git fetch origin