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
Add arc.land.update.default config for setting the default strategy

Summary:
This adds a arcconfig setting to allow specifying whether to use the merge
or rebase strategy when doing the feature branch update.
arc.land.update.default can be set to either 'rebase' or 'merge'.  The command
line flags will override this setting.

We have had trouble with arc land producing merge commits (introduced
with D4080) in git. They usually appear when arc land fails, and our users
are confused by the presence of a merge commit afterwards. Today it got even
worse since a user managed to get arc land to push the merge commit to the
server. This setting will allow us to turn it off for our uses.

Test Plan:
Verified the following combinations:
update.default not set + arc land (saw git merge in the trace)
update.default = 'rebase' + arc land (saw git rebase)
update.default = 'merge' + arc land (saw git merge)
update.default = 'rebase' + arc land --update-with-merge (saw git merge)
update.default = 'merge' + arc land --update-with-rebase (saw git rebase)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5683
Merge branch 'master' of http://github.com/facebook/arcanist