Change default behaviour for conflicts

[pazembrz]

All conflicts should be displayed as changes form head. For conflicts which do not have separate configuration If there is conflict on update, then HEAD value should be in merged and update should be put as conflict. So when curator reject all conflicts only HEAD values should be left. This will require to set default fallback conflict strategy to KEEP_HEAD.
All conflicts which we are ignoring (ones from conflict_filters) should stay with HEAD values if they are not configured differently.

[michamos]

I don't think you need to set the default to KEEP_HEAD. It can stay as it is, but non-ignored conflicts (those not in conflict_filters) should have KEEP_HEAD always.