Change fileName to filename

Summary: There are lots of callsites to $changeset->getFilename() so it seemed
easier to rename getFileName() to getFilename() even if it includes database
change. Plus I think that getFilename() is better.

Test Plan:
Alter database.
Open revision.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D1437