Merge branch 'jt/fast-import-signed-modes'

Merge branch 'jt/fast-import-signed-modes'

Handling of signed commits and tags in fast-import has been made more
configurable.

* jt/fast-import-signed-modes:
  fast-import: add 'abort-if-invalid' mode to '--signed-tags=<mode>'
  fast-import: add 'sign-if-invalid' mode to '--signed-tags=<mode>'
  fast-import: add 'strip-if-invalid' mode to '--signed-tags=<mode>'
  fast-import: add 'abort-if-invalid' mode to '--signed-commits=<mode>'
  fast-export: check for unsupported signing modes earlier