plumbing: support SSH/X509 signed tags

This commit enables support for extracting the SSH and X509 signatures
from (annotated) Git tags, as an initial step to support the
verification of more signatures than just PGP in go-git.

The ported logic from Git further ensures that we look for a signature
at the tail of an annotation, instead of the first signature we find
in the annotation, as this could theoretically result in a faulty
signature getting detected if part of a an annotation itself (e.g.
by being placed in the middle as part of an inherited message).

For commits, no further change is required as the current extraction
of any signature (format) from `gpgsig` in the commit header is
sufficient for manual verification.

In a future iteration, we could add `signature/ssh` and `signature/x509`
packages to further enable people to deal with verifying other
signatures than PGP. As well as adding additional methods to `Commit`
and `Tag` to provide glue between the packages and the most prominent
user-facing APIs.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

0 files changed, 0 insertions, 0 deletions