Having a way of amending the working copy (or part of it) into an
ancestor commit is one of the most requested features from our users
at work. This patch adds an very early version of such a command. So
far the command just creates a temporary commit and hooks into `hg
abort` to uncommit the temporary commit. Later versions will rebase
the temporary commit onto the target commit, fold it into that commit,
and then rebase the rest of the stack on top.
I called the flag -r/--rev instead of --into because I was
concerned that hg amend --into might be confused with hg amend -i.