hook: have a generic hook for transaction opening
We are adding generic hooking for all transactions. We do not really have any
useful information to include when opening the transaction but this is a
useful time to allow a hook anyway. We better let people abort transaction before
they happen than after multiple seconds/minutes of processing.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a