core: Cleanup commitpartial file with fd-relative lookups

First, this is just a general continuation of the `GFile -> openat`
transition.

Second, it's preparatory work for fsck to gain awareness of partial
commits.

src/libostree/ostree-core-private.h

@@ -112,6 +112,12 @@ _ostree_get_relative_static_delta_part_path (const char        *from,
                                              const char        *to,
                                              guint              i);
 
+static inline char *
+_ostree_get_commitpartial_path (const char *checksum)
+{
+  return g_strconcat ("state/", checksum, ".commitpartial", NULL);
+}
+
 void
 _ostree_loose_path (char              *buf,
                     const char        *checksum,