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-branch.txt

@@ -11,8 +11,8 @@ SYNOPSIS
 'git branch' [--color[=<when>] | --no-color] [--show-current]
 	[-v [--abbrev=<length> | --no-abbrev]]
 	[--column[=<options>] | --no-column] [--sort=<key>]
-	[(--merged | --no-merged) [<commit>]]
-	[--contains [<commit]] [--no-contains [<commit>]]
+	[--merged [<commit>]] [--no-merged [<commit>]]
+	[--contains [<commit>]] [--no-contains [<commit>]]
 	[--points-at <object>] [--format=<format>]
 	[(-r | --remotes) | (-a | --all)]
 	[--list] [<pattern>...]
@@ -252,13 +252,11 @@ start-point is either a local or remote-tracking branch.
 
 --merged [<commit>]::
 	Only list branches whose tips are reachable from the
-	specified commit (HEAD if not specified). Implies `--list`,
-	incompatible with `--no-merged`.
+	specified commit (HEAD if not specified). Implies `--list`.
 
 --no-merged [<commit>]::
 	Only list branches whose tips are not reachable from the
-	specified commit (HEAD if not specified). Implies `--list`,
-	incompatible with `--merged`.
+	specified commit (HEAD if not specified). Implies `--list`.
 
 <branchname>::
 	The name of the branch to create or delete.
@@ -370,6 +368,8 @@ serve four related but different purposes:
 - `--no-merged` is used to find branches which are candidates for merging
   into HEAD, since those branches are not fully contained by HEAD.
 
+include::ref-reachability-filters.txt[]
+
 SEE ALSO
 --------
 linkgit:git-check-ref-format[1],