Add new "git replace" command

Add new "git replace" command

This command can only be used now to list replace refs in
"refs/replace/" and to delete them.

The option to list replace refs is "-l".
The option to delete replace refs is "-d".

The behavior should be consistent with how "git tag" and "git branch"
are working.

The code has been copied from "builtin-tag.c" by Kristian Høgsberg
<krh@redhat.com> and Carlos Rica <jasampler@gmail.com> that was itself
based on git-tag.sh and mktag.c by Linus Torvalds.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>