git-p4: better message for "git-p4 sync" when not cloned

A common error is to do "git-p4 sync" in a repository that
was not initialized by "git-p4 clone".  There will be no
p4 refs.  The error message in this case is a traceback
for an assertion, which is confusing.

Change it instead to explain the likely problem.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Acked-By: Tor Arvid Lund <torarvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -1676,6 +1676,8 @@ class P4Sync(Command):
 
                 changes.sort()
             else:
+                if 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),
                                                               self.changeRange)