Merge branch 'ao/check-resolve-ref-unsafe-result'

"git commit", after making a commit, did not check for errors when
asking on what branch it made the commit, which has been correted.

* ao/check-resolve-ref-unsafe-result:
  commit: check result of resolve_ref_unsafe

builtin/commit.c

@@ -1492,6 +1492,8 @@ static void print_summary(const char *prefix, const struct object_id *oid,
 	diff_setup_done(&rev.diffopt);
 
 	head = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
+	if (!head)
+		die_errno(_("unable to resolve HEAD after creating commit"));
 	if (!strcmp(head, "HEAD"))
 		head = _("detached HEAD");
 	else