merge: add stubs for preprocess and conclude steps of merge driver
The exact semantics for what should happen (particularly with respect to error
handling) are still a bit hard to pin down, so I think it's better to
experiment with it as an extension for now. For now this stub will act as a
convenient point for extensions to hook on.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown