Exclude the HEAD symbolic ref from the list of known branches

Signed-off-by: Marius Storm-Olsen <mstormo_git@storm-olsen.com>

contrib/fast-import/git-p4

@@ -934,7 +934,7 @@ class P4Sync(Command):
             line = line.strip()
 
             ## only import to p4/
-            if not line.startswith('p4/'):
+            if not line.startswith('p4/') or line == "p4/HEAD":
                 continue
             branch = line
             if self.importIntoRemotes: