status: show branchname with a configurable color

You can tell "git status" to paint the name of the current branch in its
output (the line that says "On branch ...") by setting the configuration
variable color.status.branch; it is by default turned off.

Signed-off-by: Aleksi Aalto <aga@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 6a6c0b5..6cd762d 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -774,7 +774,8 @@
 	one of `header` (the header text of the status message),
 	`added` or `updated` (files which are added but not committed),
 	`changed` (files which are changed but not added in the index),
-	`untracked` (files which are not tracked by git), or
+	`untracked` (files which are not tracked by git),
+	`branch` (the current branch), or
 	`nobranch` (the color the 'no branch' warning is shown in, defaulting
 	to red). The values of these variables may be specified as in
 	color.branch.<slot>.