Merge branch 'jk/combine-diff-cleanup'

Code clean-up for code paths around combined diff.

* jk/combine-diff-cleanup:
  tree-diff: make list tail-passing more explicit
  tree-diff: simplify emit_path() list management
  tree-diff: use the name "tail" to refer to list tail
  tree-diff: drop list-tail argument to diff_tree_paths()
  combine-diff: drop public declaration of combine_diff_path_size()
  tree-diff: inline path_appendnew()
  tree-diff: pass whole path string to path_appendnew()
  tree-diff: drop path_appendnew() alloc optimization
  run_diff_files(): de-mystify the size of combine_diff_path struct
  diff: add a comment about combine_diff_path.parent.path
  combine-diff: use pointer for parent paths
  tree-diff: clear parent array in path_appendnew()
  combine-diff: add combine_diff_path_new()
  run_diff_files(): delay allocation of combine_diff_path