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