revset: add 'only' revset

Adds a only() revset that has two forms:

only(<set>) is equivalent to "::<set> - ::(heads() - heads(<set>::))"

only(<include>,<exclude>) is equivalent to "::<include> - ::<exclude>"

On a large repo, this implementation can process/traverse 50,000 revs in 0.7
seconds, versus 4.2 seconds using "::<include> - ::<exclude>".

This is useful for performing histedits on your branch:
hg histedit -r 'first(only(.))'

Or lifting branch foo off of branch bar:
hg rebase -d @ -s 'only(foo, bar)'

Or a variety of other uses.
setup: specify --prefix="" for install-home-bin to avoid badness with defaults

Sometimes (I'm not sure when) setup.py gets a default value for
--prefix, which breaks --home. This fixes that for me.