Consolidate BadName exception compatibility

As BadName only exists within some versions of GitPython, consolidate
it to the compatibility project module.

Change-Id: I638b68aa72cf5491358db40fa04290d41f4ee2ef

git_upstream/lib/drop.py

@@ -20,14 +20,10 @@ import re
 from git import BadObject
 
 from git_upstream.errors import GitUpstreamError
+from git_upstream.lib.pygitcompat import BadName
 from git_upstream.lib.utils import GitMixin
 from git_upstream.log import LogDedentMixin
 
-try:
-    from git import BadName
-except ImportError:
-    BadName = None
-
 
 class DropError(GitUpstreamError):
     """Exception thrown by L{Drop}"""

git_upstream/lib/pygitcompat.py

@@ -27,6 +27,11 @@ except ImportError:
     from git import Commit
     from git import Repo
 
+try:
+    from git import BadName
+except ImportError:
+    BadName = object
+
 
 class GitUpstreamCompatRepo(Repo):
 

git_upstream/lib/supersede.py

@@ -22,17 +22,12 @@ from git import Head
 
 from git_upstream.errors import GitUpstreamError
 from git_upstream.lib import note  # noqa
+from git_upstream.lib.pygitcompat import BadName
 from git_upstream.lib.searchers import CommitMessageSearcher
 from git_upstream.lib.utils import GitMixin
 from git_upstream.log import LogDedentMixin
 
 
-try:
-    from git import BadName
-except ImportError:
-    BadName = None
-
-
 class SupersedeError(GitUpstreamError):
     """Exception thrown by L{Supersede}"""
     pass