Merge branch 'jk/has-uncommitted-changes-fix'

"git pull --rebase" on a corrupt HEAD caused a segfault.  In
general we substitute an empty tree object when running the in-core
equivalent of the diff-index command, and the codepath has been
corrected to do so as well to fix this issue.

* jk/has-uncommitted-changes-fix:
  has_uncommitted_changes(): fall back to empty tree

diff-lib.c

@@ -520,6 +520,9 @@ int run_diff_index(struct rev_info *revs, int cached)
 	struct object_array_entry *ent;
 	uint64_t start = getnanotime();
 
+	if (revs->pending.nr != 1)
+		BUG("run_diff_index must be passed exactly one tree");
+
 	ent = revs->pending.objects;
 	if (diff_cache(revs, &ent->item->oid, ent->name, cached))
 		exit(128);