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.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)