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/sys/src/auto/versions.txt

@@ -1,2 +1,2 @@
 Generated by gir (https://github.com/gtk-rs/gir @ 2d1ffab)
-from gir-files (https://github.com/gtk-rs/gir-files @ b3c601f)
+from gir-files (https://github.com/gtk-rs/gir-files @ 9fe8b26)