cmdrewrite: a new `hg fixup` command

This new command can be used to amend changes to a specific revision.

Implementation wise what it basically does is:

1) commit the working directory changes
2) relocate the new commit onto the target commit
3) fold them into one.

After the run, the working directory parent will be the obsoleted
changeset created in step(1) and descendants of the target will
get orphan instability.
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