Change hg arc diff to detect up to the nearest parent with a diff

Summary:
Changes arc diff to choose the base commit as the first ancestor
that has a diff.  So if your tree looks like master->A->B->C->D, if you
have a diff on B (which will include A), when you run arc diff on D it will
only include C and D.

This makes the scenario for stacked diffs nicer.  A user can commit A, commit B,
arc diff, commit C, commit D, arc diff, arc land B, arc land D.

Test Plan:
Commit A on top of master
Commit B on top of A
arc diff
Commit C on top of B
Commit D on top of C
arc diff

Verify the second diff contains the changes in C and D, but not A and B.

hg up B
arc land --preview
Verify that arc land shows A and B

hg up D
arc land --preview
Verify that arc land shows A, B, C, and D
(arc land should be unaffected by this change.
It always tries to land the entire branch)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5639