Correctly parse git status --porcelain=2 output with filenames with spaces

Summary:
Filenames are last in git status --porcelain=2 lines; they
are not escaped in any way, despite the fields being
whitespace-delimited. explode thus happily chops apart filenames
with spaces in them, causing later git operations to operate only on
the filename up to the first space.

Split the lines into the right number of elements -- in all cases,
this is one more than the index we're using, since filenames come last.

Test Plan:
Altering a file with a space in its path, and running arc diff -a.

Added tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D19389