Use %C formatter for 'mate' editor on non-Windows hosts

Summary:
In https://secure.phabricator.com/D2055, jungejason fixed command-line
formatting for non-Windows editors that were not TextMate ('mate').

This widens the fix to also apply to TextMate.

Test Plan: Sent out this diff in with EDITOR='mate -w'.

Reviewers: jungejason, nh, epriestley

Reviewed By: epriestley

CC: epriestley, aran

Differential Revision: https://secure.phabricator.com/D2217