Tag errors coming out of WalkCommitHistory to aid debugging

git/repo.go

@@ -138,7 +138,7 @@ func (r *Repo) EachCommit(f func(*object.Commit) error) error {
 	})
 	if err != nil && err != endError {
-		return err
+		return fmt.Errorf("WalkCommitHistory: %s", err)
 	}
 	return nil