Improve error handling when reading loose refs

When repo is trying to figure out branches the repository has by
traversing refs/heads, add exception handling for readline.

Change-Id: If3b2a3720c6496f52f629aa9a2539f186d6ec882

git_refs.py

@@ -139,13 +139,15 @@ class GitRefs(object):
   def _ReadLoose1(self, path, name):
     try:
       fd = open(path, 'rb')
-      mtime = os.path.getmtime(path)
+    except:
-    except OSError:
-      return
-    except IOError:
       return
+
     try:
+      try:
+        mtime = os.path.getmtime(path)
         id = fd.readline()
+      except:
+        return
     finally:
       fd.close()