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.
*** failed to import extension badext: bit bucket overflowextension 'hgext.gpg' overrides commands: sigs sigcheck signhg help [COMMAND]show help for a command, extension, or list of commands