repo: Add concurrency=send

Ultimately a repo is just a file descriptor wrapper with some
cached data, etc.  We can send it between threads, much like how
`gio::File` is `Send`.

Motivated by trying to write to a repo from a separate thread
in https://github.com/cgwalters/ostree-container

rust-bindings/rust/conf/ostree.toml

@@ -139,6 +139,7 @@ status = "generate"
 [[object]]
 name = "OSTree.Repo"
 status = "generate"
+concurrency = "send"
     [[object.function]]
     # [MANUAL] we special-case the checksum value
     pattern = "^(write_content|write_content_async|write_metadata|write_metadata_async)$"