git-p4: correct hasBranchPrefix verbose output

The logic here was inverted, you got a message saying the file is
ignored for each file that is not ignored.

Signed-off-by: Andrew Oakley <aoakley@roku.com>
Acked-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2315,7 +2315,7 @@ class P4Sync(Command, P4UserMap):
             return True
         hasPrefix = [p for p in self.branchPrefixes
                         if p4PathStartsWith(path, p)]
-        if hasPrefix and self.verbose:
+        if not hasPrefix and self.verbose:
             print('Ignoring file outside of prefix: {0}'.format(path))
         return hasPrefix