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-fetch.txt

@@ -48,6 +48,10 @@ include::fetch-options.txt[]
 
 include::pull-fetch-param.txt[]
 
+--stdin::
+	Read refspecs, one per line, from stdin in addition to those provided
+	as arguments. The "tag <name>" format is not supported.
+
 include::urls-remotes.txt[]
 
 
@@ -255,14 +259,14 @@ refspec.
 * Using refspecs explicitly:
 +
 ------------------------------------------------
-$ git fetch origin +pu:pu maint:tmp
+$ git fetch origin +seen:seen maint:tmp
 ------------------------------------------------
 +
-This updates (or creates, as necessary) branches `pu` and `tmp` in
+This updates (or creates, as necessary) branches `seen` and `tmp` in
 the local repository by fetching from the branches (respectively)
-`pu` and `maint` from the remote repository.
+`seen` and `maint` from the remote repository.
 +
-The `pu` branch will be updated even if it does not fast-forward,
+The `seen` branch will be updated even if it does not fast-forward,
 because it is prefixed with a plus sign; `tmp` will not be.
 
 * Peek at a remote's branch, without configuring the remote in your local