merge: fix small bug with a failed merge across a rename If $HGMERGE wasn't able to fix the conflicts, we wouldn't copy f to fd, and util.set_exec wouldn't find the file.

To run the tests, do:

cd tests/
python run-tests.py

See http://www.selenic.com/mercurial/wiki/index.cgi/WritingTests for
more information on writing tests.