Don't trim indentation in arc amend

Summary:
When there is a single line Test Plan (or anything else) then arc amend puts it on the same line as label.
It is a problem with indented line (as in this diff) because next run of arc diff will trim the leading spaces.

Test Plan: arc amend

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2054