Merge branch 'jk/remote-helper-with-signed-tags'

Allows remote-helpers to declare they can handle signed tags, and
issue a warning when using those that don't.

* jk/remote-helper-with-signed-tags:
transport-helper: add 'signed-tags' capability
transport-helper: pass --signed-tags=warn-strip to fast-export
fast-export: add --signed-tags=warn-strip mode