branch: avoid unnecessary worktrees traversals

When we rename a branch ref, we need to update any worktree that have
its HEAD pointing to the branch ref being renamed, so to make it use the
new ref name.

If we know in advance that we're renaming a branch that is not currently
checked out in any worktree, we can skip this step entirely.  Let's do
it so.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -617,7 +617,7 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 				interpreted_oldname);
 	}
 
-	if (!copy &&
+	if (!copy && (oldref_usage & IS_HEAD) &&
 	    replace_each_worktree_head_symref(worktrees, oldref.buf, newref.buf,
 					      logmsg.buf))
 		die(_("Branch renamed to %s, but HEAD is not updated!"), newname);