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