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/contrib/diff-highlight/DiffHighlight.pm

@@ -72,7 +72,7 @@ sub handle_line {
 	      (?:$COLOR?\|$COLOR?[ ])* # zero or more trailing "|"
 	                         [ ]*  # trailing whitespace for merges
 	    /x) {
-	        my $graph_prefix = $&;
+		my $graph_prefix = $&;
 
 		# We must flush before setting graph indent, since the
 		# new commit may be indented differently from what we
@@ -112,7 +112,7 @@ sub handle_line {
 	# Since we can receive arbitrary input, there's no optimal
 	# place to flush. Flushing on a blank line is a heuristic that
 	# happens to match git-log output.
-	if (!length) {
+	if (/^$/) {
 		$flush_cb->();
 	}
 }