git-p4: fix sync new branch regression

e32e00dc (git-p4: better message for "git-p4 sync" when not
cloned, 2011-02-19) broke another use case, that of using
"git-p4 sync" to import a new branch into an existing repository.

Refine the fix again, on top of the fix in ac34efcf.
Reported-by: NMichael Horowitz <michael.horowitz@ieee.org>
Signed-off-by: NPete Wyckoff <pw@padd.com>
Tested-by: NMichael Horowitz <michael.horowitz@ieee.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -1694,7 +1694,9 @@ class P4Sync(Command):
 
                 changes.sort()
             else:
-                if not isinstance(self, P4Clone) and not self.p4BranchesInGit:
+                # catch "git-p4 sync" with no new branches, in a repo that
+                # does not have any existing git-p4 branches
+                if len(args) == 0 and not self.p4BranchesInGit:
                     die("No remote p4 branches.  Perhaps you never did \"git p4 clone\" in here.");
                 if self.verbose:
                     print "Getting p4 changes for %s...%s" % (', '.join(self.depotPaths),