lib: Add an API to GPG verify a commit given a remote

Conceptually we've been moving towards having our GPG verification
paths be per-remote.  The code internally supports this, but we
didn't expose an API to use it conveniently.

This came up when trying to add a new `gpgkeypath` option, since
right now rpm-ostree manually finds keyrings for the remote, and
hence it wasn't looking at the keypath, and said "Unknown key"
in status.

Adding an API fixes this nicely.

Closes: #576
Approved by: giuseppe

src/libostree/ostree-repo.c

@@ -4509,6 +4509,36 @@ ostree_repo_verify_commit_ext (OstreeRepo    *self,
                                               error);
 }
 
+/**
+ * ostree_repo_verify_commit_for_remote:
+ * @self: Repository
+ * @commit_checksum: ASCII SHA256 checksum
+ * @remote: OSTree remote to use for configuration
+ * @cancellable: Cancellable
+ * @error: Error
+ *
+ * Read GPG signature(s) on the commit named by the ASCII checksum
+ * @commit_checksum and return detailed results, based on the keyring
+ * configured for @remote.
+ *
+ * Returns: (transfer full): an #OstreeGpgVerifyResult, or %NULL on error
+ */
+OstreeGpgVerifyResult *
+ostree_repo_verify_commit_for_remote (OstreeRepo    *self,
+                                      const gchar   *commit_checksum,
+                                      const gchar   *remote_name,
+                                      GCancellable  *cancellable,
+                                      GError       **error)
+{
+  return _ostree_repo_verify_commit_internal (self,
+                                              commit_checksum,
+                                              remote_name,
+                                              NULL,
+                                              NULL,
+                                              cancellable,
+                                              error);
+}
+
 /**
  * ostree_repo_gpg_verify_data:
  * @self: Repository