histedit: move outgoing processing to its own function
Every piece of code extracted from the main command is a win. We simplify
changeset determination in the process. Parent ceases being a list before
becoming a node. We how have a root variable containing a node all the time.
basic test for hg debugrebuildstate
$ hg init repo
$ cd repo
$ touch foo bar
$ hg ci -Am 'add foo bar'
adding bar
adding foo
$ touch baz
$ hg add baz
$ hg rm bar
$ hg debugrebuildstate
state dump after
$ hg debugstate --nodates | sort
n 644 -1 bar
n 644 -1 foo
status
$ hg st -A
! bar
? baz
C foo
$ cd ..