lib: Add an "is_system" member to OstreeRepo

This is prep for introducing a fd-relative `ostree_repo_new_at()`.
Previously, `ostree_repo_is_system()` compared `GFile` paths, but
there's a much simpler check we can do first - if this repository
was created via `OstreeSysroot`, it must be a system repo.

Closes: #886
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -106,6 +106,7 @@ struct OstreeRepo {
 
   gboolean inited;
   gboolean writable;
+  gboolean is_system; /* Was this repo created via ostree_sysroot_get_repo() ? */
   GError *writable_error;
   gboolean in_transaction;
   gboolean disable_fsync;