When generating diffs in "arc diff", disable Git config option "diff.suppressBlankEmpty"

Summary:
Ref T13432. Git has a "diff.suppressBlankEmpty" config option which makes it emit nonstandard diffs with trimmed trailing whitespace on unchanged blank lines.

Currently, we don't parse these diffs correctly. Even if we do in the future, emitting a more standard diff is desirable.

Explicitly disable this option when executing "git diff" so we build more standard diffs.

Test Plan:

• Configured this option.
• Modified a file with a blank line in it without changing the blank line, got this goofy display diff:

• Applied patch, rediffed the same change, saw "-c diff.suppressBlankEmpty" in "--trace" output and got this sensible diff:

Maniphest Tasks: T13432

Differential Revision: https://secure.phabricator.com/D20877