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-shortlog.txt

@@ -47,9 +47,38 @@ OPTIONS
 
 	Each pretty-printed commit will be rewrapped before it is shown.
 
+--group=<type>::
+	Group commits based on `<type>`. If no `--group` option is
+	specified, the default is `author`. `<type>` is one of:
++
+--
+ - `author`, commits are grouped by author
+ - `committer`, commits are grouped by committer (the same as `-c`)
+ - `trailer:<field>`, the `<field>` is interpreted as a case-insensitive
+   commit message trailer (see linkgit:git-interpret-trailers[1]). For
+   example, if your project uses `Reviewed-by` trailers, you might want
+   to see who has been reviewing with
+   `git shortlog -ns --group=trailer:reviewed-by`.
++
+Note that commits that do not include the trailer will not be counted.
+Likewise, commits with multiple trailers (e.g., multiple signoffs) may
+be counted more than once (but only once per unique trailer value in
+that commit).
++
+Shortlog will attempt to parse each trailer value as a `name <email>`
+identity. If successful, the mailmap is applied and the email is omitted
+unless the `--email` option is specified. If the value cannot be parsed
+as an identity, it will be taken literally and completely.
+--
++
+If `--group` is specified multiple times, commits are counted under each
+value (but again, only once per unique value in that commit). For
+example, `git shortlog --group=author --group=trailer:co-authored-by`
+counts both authors and co-authors.
+
 -c::
 --committer::
-	Collect and show committer identities instead of authors.
+	This is an alias for `--group=committer`.
 
 -w[<width>[,<indent1>[,<indent2>]]]::
 	Linewrap the output by wrapping each line at `width`.  The first
@@ -76,6 +105,9 @@ them.
 Paths may need to be prefixed with `--` to separate them from
 options or the revision range, when confusion arises.
 
+:git-shortlog: 1
+include::rev-list-options.txt[]
+
 MAPPING AUTHORS
 ---------------