Catch ValueError in BaseRepo.__getitem__ when querying object store

This allows us to fall back on querying the refs when looking up 40-character
refs.

Fixes: #82

dulwich/repo.py

@@ -1107,7 +1107,7 @@ class BaseRepo(object):
         if len(name) in (20, 40):
             try:
                 return self.object_store[name]
-            except KeyError:
+            except (KeyError, ValueError):
                 pass
         try:
             return self.object_store[self.refs[name]]

dulwich/tests/data/repos/refs.git/refs/heads/40-char-ref-aaaaaaaaaaaaaaaaaa

@@ -0,0 +1 @@
+42d06bd4b77fed026b154d16493e5deab78f02ec