repo: Introduce ostree_repo_open_at() and ostree_repo_create_at()

This essentially completes our fd-relative conversion.

While here, I cleaned up the semantics of `ostree_repo_create()` and
`ostree_repo_create_at()` to be more atomic - basically various scripts were
testing for the `objects` subdirectory, so let's formalize that.

Closes: #820
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -96,7 +96,11 @@ struct OstreeRepo {
   char *commit_stagedir_name;
   GLnxLockFile commit_stagedir_lock;
 
-  GFile *repodir;
+  /* A cached fd-relative version, distinct from the case where we may have a
+   * user-provided absolute path.
+   */
+  GFile *repodir_fdrel;
+  GFile *repodir; /* May be %NULL if we were opened via ostree_repo_open_at() */
   int    repo_dir_fd;
   int    tmp_dir_fd;
   int    cache_dir_fd;
@@ -132,10 +136,13 @@ struct OstreeRepo {
   GHashTable *updated_uncompressed_dirs;
   GHashTable *object_sizes;
 
-  uid_t owner_uid;
-  uid_t target_owner_uid;
+  /* Cache the repo's device/inode to use for comparisons elsewhere */
+  dev_t device;
+  ino_t inode;
+  uid_t owner_uid; /* Cache of repo's owner uid */
+  uid_t target_owner_uid; /* Ensure files are chowned to this uid/gid */
   gid_t target_owner_gid;
-  guint min_free_space_percent;
+  guint min_free_space_percent; /* See the min-free-space-percent config option */
 
   guint test_error_flags; /* OstreeRepoTestErrorFlags */