Merge branch 'tc/diff-tree-max-depth'

"git diff-tree" learned "--max-depth" option.

* tc/diff-tree-max-depth:
  diff: teach tree-diff a max-depth parameter
  within_depth: fix return for empty path
  combine-diff: zero memory used for callback filepairs

diff --cc Documentation/diff-options.adoc
Simple merge

diff --cc Makefile
Simple merge

diff --cc combine-diff.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge

diff --cc dir.c
Simple merge

diff --cc t/meson.build
Simple merge