Use general git revisions as arguments to commits

[geigerzaehler]

It would be great if radicle_source::commits would behave more like the git rev-list command and accept arbitrary git revisions. This would allow for more advanced use cases to get commit histories like differences between branches. (See also radicle-dev/radicle-upstream#1737).

One approach to this would be to modify radicle_source::Revsion so that the user is able to encode options to select a range of commits. A comprehensive solution would need to be able to encode all possible git revisions. Another approach could be to leverage git2::Repository::revparse which returns a Revspec.