Documentation/git-branch: add default for --contains

Indicate that the commit parameter of --contains defaults to HEAD.

Signed-off-by: Vincent van Ravesteijn <vfr@lyx.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -163,8 +163,9 @@ start-point is either a local or remote-tracking branch.
 	Open an editor and edit the text to explain what the branch is
 	for, to be used by various other commands (e.g. `request-pull`).
 
---contains <commit>::
-	Only list branches which contain the specified commit.
+--contains [<commit>]::
+	Only list branches which contain the specified commit (HEAD
+	if not specified).
 
 --merged [<commit>]::
 	Only list branches whose tips are reachable from the