deltas: Search for similar objects (possibly renamed across directories)

The previous diff algorithm was file tree based, and only looked
at modified files that lived at the same path.

However, components like the Linux kernel have versioned
subdirectories, e.g. /usr/lib/modules/$kver/.../ext4.ko.  We want to
be able to detect these "modified renames" so that we can compute
diffs (rollsum, bsdiff).

src/libostree/ostree-repo-static-delta-private.h

@@ -152,4 +152,23 @@ _ostree_repo_static_delta_part_have_all_objects (OstreeRepo             *repo,
                                                  gboolean               *out_have_all,
                                                  GCancellable           *cancellable,
                                                  GError                **error);
+
+typedef struct {
+  char *checksum;
+  guint64 size;
+  GPtrArray *basenames;
+} OstreeDeltaContentSizeNames;
+
+void _ostree_delta_content_sizenames_free (gpointer v);
+
+gboolean
+_ostree_delta_compute_similar_objects (OstreeRepo                 *repo,
+                                       GVariant                   *from_commit,
+                                       GVariant                   *to_commit,
+                                       GHashTable                 *new_reachable_regfile_content,
+                                       guint                       similarity_percent_threshold,
+                                       GHashTable                **out_modified_regfile_content,
+                                       GCancellable               *cancellable,
+                                       GError                    **error);
+
 G_END_DECLS