First stab at a revert command.

Needs tests.
Remove debugging code, add TODO notes.
Revision ids weren't being lowercased because I forgot to coerece.
Use get_revision_maybe().
Add --dry-run option to the revert command.
Merge branch 'master' into feature/revert
Mention new revert command in change log.
Mention PINTO_EDITOR variable in docs for the --message option.
POD formatting only.
Just whitespace.
Prevent trying to revert past the root.
Test cases for the revert command.
Merge branch 'feature/revert'