git-p4: document and test clone --branch

Clone with --branch will not checkout HEAD, unless the branch
happens to be called the default refs/remotes/p4/master.  The
--branch option is most useful with sync; give an example of
that.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-p4.txt

@@ -178,7 +178,15 @@ subsequent 'sync' operations.
 --branch <branch>::
 	Import changes into given branch.  If the branch starts with
 	'refs/', it will be used as is, otherwise the path 'refs/heads/'
-	will be prepended.  The default branch is 'master'.
+	will be prepended.  The default branch is 'master'.  If used
+	with an initial clone, no HEAD will be checked out.
++
+This example imports a new remote "p4/proj2" into an existing
+git repository:
+----
+    $ git init
+    $ git p4 sync --branch=refs/remotes/p4/proj2 //depot/proj2
+----
 
 --detect-branches::
 	Use the branch detection algorithm to find new paths in p4.  It is

t/t9806-git-p4-options.sh

@@ -27,6 +27,17 @@ test_expect_success 'clone no --git-dir' '
 	test_must_fail "$GITP4" clone --git-dir=xx //depot
 '
 
+test_expect_success 'clone --branch' '
+	"$GITP4" clone --branch=refs/remotes/p4/sb --dest="$git" //depot &&
+	test_when_finished cleanup_git &&
+	(
+		cd "$git" &&
+		git ls-files >files &&
+		test_line_count = 0 files &&
+		test_path_is_file .git/refs/remotes/p4/sb
+	)
+'
+
 test_expect_success 'kill p4d' '
 	kill_p4d
 '