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 da/foo
adding foo
% manifest
200 Script output follows


drwxr-xr-x da
-rw-r--r-- 4 foo


200 Script output follows


-rw-r--r-- 4 foo