avoid claiming that the source of a file move was unchanged

Summary:
sometimes we show moved files as unchanged, sometimes deleted,
and sometimes inconsistent with what actually happened at the
destination of the move. Rather than make a false claim,
omit the 'not changed' notice if this is the source of a move.

Background: one of our users was confused by the source of a move
claiming to be unmodified when the destination of the move had
extensive modifications.

There may be a question about the quality of the data we keep/compute
about the changes, so maybe we could do a better or more consistent
job there (may just be nuances of the SCM in use); this approach
just smoothes that out and doesn't require fixing up the status
in pre-existing diffs.

Test Plan:
In the facebook phabricator, D594195#739ace1a and D590020#b97cfcfd
For others, find a diff that has moved files.
For the source of a move, we should now only show the "X moved to Y"
header and not the "unchanged" shield.

Reviewers: nh, vrana, epriestley

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3669