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 b
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       3      0       0 1e88685f5dde 000000000000 000000000000