Only associate branch with task if maniphest is enabled

Summary:
Previously, maniphest tasks would get upated by diffs on branches
with tasky names, even if maniphest was disabled.

Test Plan:
Tested createing a diff in sandbox with maniphest disabled, on a
git branch named using the format "t###".  Without this change,
if there happened to be a task in the maniphest DB which matched,
it was updated an email was sent to users.

Reviewers: epriestley

Reviewed By: epriestley

CC: wez, slawekbiel, whhone, Korvin, aran

Differential Revision: https://secure.phabricator.com/D6881