Allow "differential.getcommitmessage" to be called without a revision ID in order to generate a template

Summary: See T614. This allows us to generate an empty template by calling
Conduit, so we can build command-line editing workflows for SVN, Mercurial, and
conservative-Git.

Test Plan: Used web console to invoke Conduit method; got a reasonable empty
template out of it.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley, btrahan

Differential Revision: 1156