When specifying a `patch.base` to point to for stacking patches, it's
useful to accept any kind of revspec, e.g. `HEAD^`.
Change the argument parsing to use the `Rev` type instead, and change
the `rad-patch-ahead-behind` to use `HEAD^` to test the example usage.
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett