lib/repo: More cleanup of load_file() internals

This is followon work from previous cleanups.  Basically
`stat_bare_content_object()` was the `fstatat()` logic
and `ostree_repo_read_bare_fd()` was the `openat()` implementation;
they duplicated some bits to find the object in staging, recurse
into parent etc.

Further, I wanted an internal-only version of this API which didn't allocate
`GFileInfo`/`GInputStream` but used a plain `fd` and `struct stat` to avoid
mallocs.

The end version here I think looks a lot nicer, since we deduplicate the various
`open()` calls in the different cases for example.

Closes: #952
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -321,11 +321,14 @@ _ostree_repo_commit_trusted_content_bare (OstreeRepo          *self,
                                           GError             **error);
 
 gboolean
-_ostree_repo_read_bare_fd (OstreeRepo           *self,
-                           const char           *checksum,
-                           int                  *out_fd,
-                           GCancellable        *cancellable,
-                           GError             **error);
+_ostree_repo_load_file_bare (OstreeRepo         *self,
+                             const char         *checksum,
+                             int                *out_fd,
+                             struct stat        *out_stbuf,
+                             char              **out_symlink,
+                             GVariant          **out_xattrs,
+                             GCancellable       *cancellable,
+                             GError            **error);
 
 gboolean
 _ostree_repo_update_mtime (OstreeRepo        *self,