revset: add hook after tree parsing
This will be useful to execute actions after the tree is parsed and
before the revset returns a match. Finding symbols in the parse tree
will later allow hashes of hidden revisions to work on the command
line without the --hidden flag.
$ 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