Don't corrupt commit message in arc amend

Summary:
I use a smart editor which wraps words by itself so that I don't need them to be
wrapped by actual newlines.
Curent state disallows me adding or removing words later without uglying the
formatting.
Also the wordwrapped message looks ugly in Phabricator.
I am not sure how the commit message would look like on other places (such as
GitHub) but all reasonable tools should be able to wrap the text by itself.

Test Plan: arc amend --show # on a diff with long lines

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1755