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.
http://mercurial.selenic.com/bts/issue1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..