Use gitpython to find a commit reference

Fixes #3842

Author: agjohnson

readthedocs/vcs_support/backends/git.py

@@ -10,7 +10,8 @@
 import re
 
 from django.core.exceptions import ValidationError
-import git
+from git import Repo
+from git.exc import BadName
 from six import PY2, StringIO
 
 from readthedocs.core.validators import validate_repository_url
@@ -273,8 +274,13 @@ def find_ref(self, ref):
         return ref
 
     def ref_exists(self, ref):
-        code, _, _ = self.run('git', 'show-ref', ref, record_as_success=True)
-        return code == 0
+        try:
+            r = git.Repo(self.working_dir)
+            if r.commit(ref):
+                return True
+        except BadName:
+            return False
+        return False
 
     @property
     def env(self):

requirements/pip.txt

@@ -47,7 +47,7 @@ dnspython==1.15.0
 
 # VCS
 httplib2==0.10.3
-gitpython==2.1.8
+GitPython==2.1.8
 
 # Search
 elasticsearch==1.5.0