Don't prepopulate update message in hg amend diff workflow

Summary:
Previously, updating a commit via arc diff in mercurial would
prepopulate the update message with part of the commit message.  In an
amend workflow this doesn't make sense, so I disabled it. Git already
does this, so now mercurial matches git in this scenario.

We had users complain that new users would often submit diffs with the
default update message, and it wasn't useful since they were using a
amend flow.

Test Plan:
arc diff on a commit that already had a diff
Verified the editor did not have a update message
arc diff on a stack of commits where the bottom one had a diff
Verified the editor provided a default update message

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6215