git-p4: fix clone @all regression

e32e00d (git-p4: better message for "git-p4 sync" when not cloned,
2011-02-19) broke the use of the "@all" revision specifier, e.g.,

    git-p4 clone //depot/xxx@all

Fix it as per Tor Arvid's quick patch.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Reported-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Based-on-patch-by: Tor Arvid Lund <torarvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -1694,7 +1694,7 @@ class P4Sync(Command):
 
                 changes.sort()
             else:
-                if not self.p4BranchesInGit:
+                if not isinstance(self, P4Clone) 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),