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);

t/t5520-pull.sh

@@ -618,6 +618,18 @@ test_expect_success 'pull --rebase fails on unborn branch with staged changes' '
 	)
 '
 
+test_expect_success 'pull --rebase fails on corrupt HEAD' '
+	test_when_finished "rm -rf corrupt" &&
+	git init corrupt &&
+	(
+		cd corrupt &&
+		test_commit one &&
+		obj=$(git rev-parse --verify HEAD | sed "s#^..#&/#") &&
+		rm -f .git/objects/$obj &&
+		test_must_fail git pull --rebase
+	)
+'
+
 test_expect_success 'setup for detecting upstreamed changes' '
 	mkdir src &&
 	(cd src &&

wt-status.c

@@ -2340,7 +2340,17 @@ int has_uncommitted_changes(int ignore_submodules)
 	if (ignore_submodules)
 		rev_info.diffopt.flags.ignore_submodules = 1;
 	rev_info.diffopt.flags.quick = 1;
+
 	add_head_to_pending(&rev_info);
+	if (!rev_info.pending.nr) {
+		/*
+		 * We have no head (or it's corrupt); use the empty tree,
+		 * which will complain if the index is non-empty.
+		 */
+		struct tree *tree = lookup_tree(the_hash_algo->empty_tree);
+		add_pending_object(&rev_info, &tree->object, "");
+	}
+
 	diff_setup_done(&rev_info.diffopt);
 	result = run_diff_index(&rev_info, 1);
 	return diff_result_code(&rev_info.diffopt, result);