Merge branch 'rs/branch-allow-deleting-dangling'

"git branch -D <branch>" used to refuse to remove a broken branch
ref that points at a missing commit, which has been corrected.

* rs/branch-allow-deleting-dangling:
  branch: allow deleting dangling branches with --force

builtin/branch.c

@@ -168,7 +168,7 @@ static int check_branch_commit(const char *branchname, const char *refname,
 			       int kinds, int force)
 {
 	struct commit *rev = lookup_commit_reference(the_repository, oid);
-	if (!rev) {
+	if (!force && !rev) {
 		error(_("Couldn't look up commit object for '%s'"), refname);
 		return -1;
 	}