Make arc land list the commits being landed.

Summary:
Arc land is a bit magical and some users have gotten bitten by
the fact that it collapses and lands every commit on the branch. To make
it explicit what is being landed, it now shows a list of the commits
that are being landed.  I also added a --preview flag that will just
print the commits that would be landed, but does nothing else.

Hopefully this make arc land a little less magical for people.

Test Plan:
arc land in the following scenarios:
- Landing one change
- Landing no changes
- Landing a stack of changes

Did it with hg and git.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, sid0, dschleimer, bos, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5460