other than just removing a key, which doesn't affect the verified state
of a commit, revoking a key unverifies all signed commits.

since everything (except the CurrentKeyChain method) operates on a
tempoary keychain anyway we don't need this anymore.

According to https://github.com/twbs/bootstrap/issues/10547 it's not
possible to have the trigger defined on the delegated element, i.e. not
defined as a data attribute.

the rugged versions up to 0.26.0b3 had a bug concerning the signature
extraction. The extracted signature was not always the same, probably
due to a buffer (overflow) issue in libgit.

see https://github.com/libgit2/rugged/issues/608

we introduced memoizing, so it's safe to call the method multiple times.