repo: Reject duplicate signatures when signing commit
repo: Reject duplicate signatures when signing commit

Uses OstreeGpgVerifyResult to catch duplicate signatures.

If the commit has already been signed with the given GPG key ID, fail
with a G_IO_ERROR_EXISTS error code.

leandrosansilva/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,resin-io/ostree,sujitfulse/ostree,leandrosansilva/ostree,resin-io/ostree,sujitfulse/ostree,leandrosansilva/ostree,GNOME/ostree,resin-io/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,resin-io/ostree,gatispaeglis/ostree,sujitfulse/ostree,sujitfulse/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,leandrosansilva/ostree