Add a --signed-commits flag in fast-export

git-fast-export(1) will strip commit signatures even if the contents of the commit doesn't change. See for context. This is a problem for our feature to remove blobs, because git-filter-repo which calls git-fast-import and git-fast-export will remove signatures across the whole repository.

Add a --signed-commits flag in fast-export so that we can also fix git-filter-repo to not strip signatures. https://public-inbox.org/git/xmqq1rao9zev.fsf@gitster.g/ contains an effort to do this, and it looks like it's most of the way there.