`http://en.wikipedia.org/wiki/Ent_(Middle-earth)` for an in-depth
explanation. Avoid this term, not to confuse people.
+ An evil merge is a <<def_merge,merge>> that introduces changes that
+ do not appear in any <<def_parent,parent>>.
A fast-forward is a special type of <<def_merge,merge>> where you have a
<<def_revision,revision>> and you are "merging" another