clone: add --branch option to select a different HEAD

We currently point the HEAD of a newly cloned repo to the
same ref as the parent repo's HEAD. While a user can then
"git checkout -b foo origin/foo" whichever branch they
choose, it is more convenient and more efficient to tell
clone which branch you want in the first place.

Based on a patch by Kirill A. Korinskiy.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 2c63a0f..1cd1ecc 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -127,6 +127,13 @@
 	Instead of using the remote name 'origin' to keep track
 	of the upstream repository, use <name>.
 
+--branch <name>::
+-b <name>::
+	Instead of pointing the newly created HEAD to the branch pointed
+	to by the cloned repositoroy's HEAD, point to <name> branch
+	instead. In a non-bare repository, this is the branch that will
+	be checked out.
+
 --upload-pack <upload-pack>::
 -u <upload-pack>::
 	When given, and the repository to clone from is accessed