patch: be more tolerant to diffstat failures (issue 865)

Sometimes, revisions cannot be represented by a regular diff, only a git diff
would capture binary files or permission changes. diffstat cannot handle git
patches and will output "0 files changed" when fed with an empty diff. We
cannot consider the latter to be an error, unless we rewrite diffstat to handle
these correctly.

mercurial/patch.py

@@ -1351,7 +1351,6 b' def diffstat(patchlines):'
             last = stat.pop()
             stat.insert(0, last)
             stat = ''.join(stat)
-            if stat.startswith('0 files'): raise ValueError
             return stat
         except: raise
     finally: