ArcanistGitAPI: remove a bogus comment

See: <https://github.com/facebook/arcanist/commit/628de7d7a15cd7b5737515ec2831b3b4c3f601a5#commitcomment-4675543>

This comment is indecipherable even if it originally meant something.

Reviewed by: epriestley
ArcanistGitAPI: document a git ls-files bug

When a submodule is ignored (ignore=all in .gitconfig),

  $ git ls-files -m

fails to exclude the submodule from the listing. Other commands like

  $ git diff-index --name-only HEAD

exclude it just fine.

See: <https://github.com/facebook/arcanist/pull/120>

Reviewed by: epriestley
ArcanistGitAPI: replace 'ls-files -m' with diff-files

As documented,

  $ git ls-files -m

fails to exclude ignored submodules. Replace it with an equivalent:

  $ git diff-files --name-only

which does not suffer from this defect.

See: <https://github.com/facebook/arcanist/pull/121>

Reviewed by: epriestley
Fix issues in C# unit test engine

Summary: This fixes a few issues in the C# unit test engine.  It fixes tests sitting in subdirectories not being tested correctly (the location of both the test assembly and the results file would be wrong).  It also fixes a very strange issue where xUnit.NET seems to not output the resulting XML file when it executes; in this case we just retry running the test until the XML file appears after completion (and eventually it works).

Test Plan: Ran `arc unit --everything` and `arc unit --everything --no-coverage` and verified that it's all reliably working.

Reviewers: epriestley, #blessed_reviewers, hach-que

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7594