fast-import: start controlling how commit signatures are handled

Tools like git-filter-repo should be able to control how commit signatures are handled when regenerating repository content after it has been filtered (see https://github.com/newren/git-filter-repo/issues/139). For this purpose, they need a way for git fast-import to control how commit signatures are handled.

This small patch series starts to implement such a way by adding a new --signed-commits=<mode> option to git fast-import.

For now this new option behaves in a very similar way as the option with the same name that already exists in git fast-export. Especially it supports exactly the same s and the same aliases for these modes. For example "ignore" is a synonym for "verbatim".

In the future I want to implement new s like "strip-if-invalid", "re-sign", "re-sign-if-invalid" that might be a bit more complex for this option. But for now I prefer to start with the simple modes to validate the general design of the new option.

In particular, I am interested in feedback about the following:

• Should we keep "ignore" as a synonym for "verbatim" and "warn" as a synonym for "warn-verbatim"? My opinion is that they might be confusing, so we might want to remove them for git fast-import even if we keep them for git fast-export. The parsing code might be a bit more complex if we do that though, so for now I have kept the synonyms.

• Are we still fine with most s having a "warn-*" variant (like the "warn-strip" variant of "strip" for example)? Or should we have a separate --verbose or maybe --signed-commits-verbose option dedicated to switching warnings on/off? I think it's good to decide about this before the number of s increases a lot with new s like "strip-if-invalid", "re-sign", "re-sign-if-invalid" and possibly others.

This is related to #519