git-rev-list: add "end" commit and "--header" flag

git-rev-list: add "end" commit and "--header" flag

The "end" commit is just faking it right now, it's sorting things
purely by date, so this is _not_ a reachability analysis. Some day.

The "--header" flag causes the commit message to be printed out,
with a NUL character separator after it for parseability. This
allows you to do things like use "grep -z" to grep for certain
authors etc.