Make the Arcanist comment remover less aggressive about stripping instructional comments

Summary:
Ref T13098. See PHI858. If you write this at the end of a message in arc diff:

Subscribers:
#projectname

# NEW DIFFERENTIAL REVISION
# Describe the changes in this new revision.
# ...

...we'll currently eat the #projectname as an instructional comment, even if it is followed by an empty line.

Instead, stop eating stuff once we hit the first empty line. (We escape empty lines in comments already.)

After T13098 I'll maybe adjust this to use a more explicit instruction escape, like ##, since there's no reason we're bound to #.

Test Plan: Added a unit test and made it pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19639