The conflict markers created by :mergediff were not detected as
conflicts, which affects both commands.resolve.mark-check and
mergetools.<tool>.check. This patch fixes that.
The new regex it uses for finding conflict markers is less restrictive
because it :mergediff doesn't follow the <<<<<<< and >>>>>>>
lines by a space (and a description). Hopefully lines like that don't
give too many false positives. We can add back the space and make
:mergediff add trailing spaces if it turns out to be a
problem. OTOH, there will always be some false positives and we have
ways of overriding the checks already.
This patch can go onto the default or stable branch, depending on how
much we care about an experimental feature.