Allow hg arc land when no rebase is necessary

Summary:
Previously, trying to arc land in a mercurial repo would
fail if the local branch was already at the tip of the onto branch
(since hg rebase exited with code 1).  This change makes it check
if a rebase is needed before executing the rebase.

Test Plan:
hg init foo
cd foo
hg bookmark master
touch a && hg add a && hg commit -ma
// setup your .arcconfig
cd ..
hg clone foo foo2
cd foo2
hg bookmark mybook
touch b && hg add b && hg commit -mb
arc land --onto master --revision <your rev number>

Arc land should succeed.  I also tried landing when a rebase was
necessary and it still worked.

Reviewers: epriestley, dschleimer, bos

Reviewed By: epriestley

CC: sid0, aran, Korvin

Differential Revision: https://secure.phabricator.com/D4588
Don't use xhpast binary to compute xhpastlinter cache version if the binary doesn't exist

Summary: The binary may not be built, in which case this raises a warning.

Test Plan: Will make @zeeg test.

Reviewers: zeeg, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4569
Added support for hg diffs generated in git mode

Summary: Fixes T2112. These are fairly common now, and are used as the storage format for `hg export` and mq in most installs.

Test Plan: Ran unit tests, used `arc patch --patch`, uploaded some diffs manually.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2112

Differential Revision: https://secure.phabricator.com/D4592
Merge branch 'master' of http://github.com/facebook/arcanist