hgmerge: do not use file with markers in tools merging themselves
This time I forgot about a crucial part of the logic change.
The patch makes kdiff3 and tkdiff useful again.
1:3aa14bbc23d90:8633637036c13aa14bbc23d9 tip8633637036c18633637036c1+8633637036c13aa14bbc23d9 tipmerging file13aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip