Summary:
the nuclide team would like to be able to get the commit template
message without actually invoking a commit. This small extensions allows them
to do this.
Caveats: ignores subrepositories.
Test Plan: new test
Reviewers: #mercurial, most
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D3960557
Tasks: 12771006
