Informative error message on not being able to locate GNU diff for Pe…

…rforce

Currently, if GNU diff is not in the path, post-review gives an unhandled
exception and a call stack. It scares the user, and left him without any clue
what is wrong. I found this is solved in case of SVN so I ported that solution
to Perforce too.

Fixes bug 2352
Reviewed at http://reviews.reviewboard.org/r/2685/

rbtools/postreview.py

@@ -1884,6 +1884,10 @@ def get_repository_info(self):
                       data, re.M)
         self.p4d_version = int(m.group(1)), int(m.group(2))
 
+        # Now that we know it's Perforce, make sure we have GNU diff installed,
+        # and error out if we don't.
+        check_gnu_diff()
+
         return RepositoryInfo(path=repository_path, supports_changesets=True)
 
     def scan_for_server(self, repository_info):