meta: git controls for text changelogs; github controls

meta: git controls for text changelogs; github controls

For the ChangeLog and files like it, use `merge=union` to bring in
content from both sides instead of having conflicts block merges because
someone else added a feature.

For GitHub, provide some "templates" which really just point people in
the right direction, but if the repointing fails, at least reduces the
pain a little.