git-p4: don't use name-rev to get current branch

git-p4 was using "git name-rev" to find out the current branch.

That is not safe, since if multiple branches or tags point at
the same revision, the result obtained might not be what is
expected.

Instead use "git symbolic-ref".

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -602,12 +602,7 @@ def p4Where(depotPath):
     return clientPath
 
 def currentGitBranch():
-    retcode = system(["git", "symbolic-ref", "-q", "HEAD"], ignore_error=True)
-    if retcode != 0:
-        # on a detached head
-        return None
-    else:
-        return read_pipe(["git", "name-rev", "HEAD"]).split(" ")[1].strip()
+    return read_pipe_text(["git", "symbolic-ref", "--short", "-q", "HEAD"])
 
 def isValidGitDir(path):
     return git_dir(path) != None

t/t9807-git-p4-submit.sh

@@ -139,7 +139,7 @@ test_expect_success 'submit with master branch name from argv' '
 	)
 '
 
-test_expect_failure 'allow submit from branch with same revision but different name' '
+test_expect_success 'allow submit from branch with same revision but different name' '
 	test_when_finished cleanup_git &&
 	git p4 clone --dest="$git" //depot &&
 	(