R1:726a4912bd7e

Allow filtering by lint code in Diffusion

Test Plan:
/diffusion/ARC/lint/master/src/, clicked on count link.
/diffusion/ARC/browse/master/src/difference/?lint=XHP9, clicked on file name.
/diffusion/ARC/browse/master/src/difference/ArcanistDiffUtils.php?lint=, verified that all messages are displayed.
/diffusion/ARC/browse/master/src/difference/ArcanistDiffUtils.php?lint=XHP9.
/diffusion/ARC/browse/master/src/difference/ArcanistDiffUtils.php?lint=TXT3, verified that 0 messages are displayed.

Reviewers…