Add a separate type for the identity in a push certificate

Add a separate type for the identity in a push certificate

These differ subtly from a PersonIdent, because they can contain
anything that is a valid User ID passed to gpg --local-user. Upstream
git push --signed will just take the configuration value from
user.signingkey and pass that verbatim in both --local-user and the
pusher field of the certificate. This does not necessarily contain an
email address, which means the parsing implementation ends up being
substantially different from RawParseUtils.parsePersonIdent.
Nonetheless, we try hard to match PersonIdent behavior in
questionable cases.

Change-Id: I37714ce7372ccf554b24ddbff56aa61f0b19cbae