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/receive.txt

@@ -114,6 +114,28 @@ receive.hideRefs::
 	An attempt to update or delete a hidden ref by `git push` is
 	rejected.
 
+receive.procReceiveRefs::
+	This is a multi-valued variable that defines reference prefixes
+	to match the commands in `receive-pack`.  Commands matching the
+	prefixes will be executed by an external hook "proc-receive",
+	instead of the internal `execute_commands` function.  If this
+	variable is not defined, the "proc-receive" hook will never be
+	used, and all commands will be executed by the internal
+	`execute_commands` function.
++
+For example, if this variable is set to "refs/for", pushing to reference
+such as "refs/for/master" will not create or update a reference named
+"refs/for/master", but may create or update a pull request directly by
+running the hook "proc-receive".
++
+Optional modifiers can be provided in the beginning of the value to filter
+commands for specific actions: create (a), modify (m), delete (d).
+A `!` can be included in the modifiers to negate the reference prefix entry.
+E.g.:
++
+	git config --system --add receive.procReceiveRefs ad:refs/heads
+	git config --system --add receive.procReceiveRefs !:refs/heads
+
 receive.updateServerInfo::
 	If set to true, git-receive-pack will run git-update-server-info
 	after receiving data from git-push and updating refs.