trydiff: simplify checking for additions
In the body of the loop in trydiff(), there are conditions like:
addedset or (f in modifiedset and to is None)
The second half of that expression is to account for the fact that
merge-in additions appear as additions. By instead fixing up the sets
of modified and added files to compensate for this fact, we can
simplify the body of the loop. It also fixes one case where the
addedset was checked without the additional check (the "have we
already reported a copy above?" case in the code, also see fixed test
case).
The similar condition with 'removedset' in it seems to have served no
purpose even before this change, so it could have been simplified even
before.