To make it more flexible and support SSH signature.
The existing tests are not changed, there are also tests covering
`parseTagRef` which also calls `parsePayloadSignature` now. Add some new
tests to `Test_parseTagData`
This module is merged from https://github.com/go-gitea/git which is a Go module to access Git through shell commands. Now it's a part of gitea's main repository for easier pull request.