unamend: move fb extension unamend to core
unamend extension adds an unamend command which undoes the effect of the amend
command. This patch moves the unamend command from that extension to uncommit
extension and this one does not completely undoes the effect of amend command as
it creates a new commit, rather than reviving the old one back.
This also adds tests for the same.
A new unamend command in uncommit extension which undoes the effect of the amend command by creating a new changeset which was there before amend and moving the changes that were amended to the working directory.
Differential Revision: https://phab.mercurial-scm.org/D821