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/config/stash.txt

@@ -1,17 +1,9 @@
 stash.useBuiltin::
-       Set to `false` to use the legacy shell script implementation of
-       linkgit:git-stash[1]. Is `true` by default, which means use
-       the built-in rewrite of it in C.
-+
-The C rewrite is first included with Git version 2.22 (and Git for Windows
-version 2.19). This option serves as an escape hatch to re-enable the
-legacy version in case any bugs are found in the rewrite. This option and
-the shell script version of linkgit:git-stash[1] will be removed in some
-future release.
-+
-If you find some reason to set this option to `false`, other than
-one-off testing, you should report the behavior difference as a bug in
-Git (see https://git-scm.com/community for details).
+	Unused configuration variable.  Used in Git versions 2.22 to
+	2.26 as an escape hatch to enable the legacy shellscript
+	implementation of stash.  Now the built-in rewrite of it in C
+	is always used. Setting this will emit a warning, to alert any
+	remaining users that setting this now does nothing.
 
 stash.showPatch::
 	If this is set to true, the `git stash show` command without an