Add new git fast-import mode to re-sign commits

In #519 a new --signed-commits=<mode> option was added to git fast-import. It has several modes including a 'strip-if-invalid' mode to check commit signatures and drop them if they are invalid.

However to be able to add new flags to git-filter-repo so it can better handle signed commits and tags as discussed in #510, we need to improve git fast-import by adding a few more modes, especially:

• a new 're-sign-if-invalid' mode to re-sign if a signature became invalid,
• a new 'abort-if-invalid' mode to error out if a signature became invalid.

Ideally, it would be nice too if the --signed-tags=<mode> option could support all the same modes as the --signed-commits=<mode> option, so that 'strip-if-invalid', 'abort-if-invalid' and 're-sign-if-invalid' work for tag signatures too.