Merge branch 'jk/diff-submodule-diff-inline'

The "git diff --submodule={short,log}" mechanism has been enhanced
to allow "--submodule=diff" to show the patch between the submodule
commits bound to the superproject.

* jk/diff-submodule-diff-inline:
  diff: teach diff to display submodule difference with an inline diff
  submodule: refactor show_submodule_summary with helper function
  submodule: convert show_submodule_summary to use struct object_id *
  allow do_submodule_path to work even if submodule isn't checked out
  diff: prepare for additional submodule formats
  graph: add support for --line-prefix on all graph-aware output
  diff.c: remove output_prefix_length field
  cache: add empty_tree_oid object and helper function

sha1_file.c

@@ -39,6 +39,12 @@ static inline uintmax_t sz_fmt(size_t s) { return s; }
 
 const unsigned char null_sha1[20];
 const struct object_id null_oid;
+const struct object_id empty_tree_oid = {
+	EMPTY_TREE_SHA1_BIN_LITERAL
+};
+const struct object_id empty_blob_oid = {
+	EMPTY_BLOB_SHA1_BIN_LITERAL
+};
 
 /*
  * This is meant to hold a *small* number of objects that you would