git mktag < signature_file
Reads a tag contents on standard input and creates a tag object that can also be used to sign other objects.
A tag signature file has a very simple fixed format: four lines of
object <sha1> type <typename> tag <tagname> tagger <tagger>
followed by some optional free-form message (some tags created by older Git may not have tagger line). The message, when exists, is separated by a blank line from the header. The message part may contain a signature that Git itself doesn't care about, but that can be verified with gpg.
Part of the git(1) suite