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.
port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export