Fix the git version parser.

Author: marcusrbrown

Diff for: git/cmd.py

@@ -236,7 +236,7 @@ def _set_cache_(self, attr):
         if attr == '_version_info':
             # We only use the first 4 numbers, as everthing else could be strings in fact (on windows)
             version_numbers = self._call_process('version').split(' ')[2]
-            self._version_info = tuple(int(n) for n in version_numbers.split('.')[:4])
+            self._version_info = tuple(int(n) for n in version_numbers.split('.')[:4] if n.isdigit())
         else:
             super(Git, self)._set_cache_(attr)
         #END handle version info