Fix ArcanistBundle to generate disjoint, minimal hunks

Summary: 'patch' chokes on hunks with too much trailing stuff. 'git apply'
chokes on overlapping hunks. Make them both happy. Write some test cases so I
stop breaking this stuff.

Test Plan:

• Applied a previously-failing patch via SVN.
• Applied a previously-failing-then-succeeding patch via Git.
• Ran unit tests.

Reviewers: jungejason, btrahan, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 1099