Merge branch 'ps/build-meson-subtree'

The meson-driven build is now aware of "git-subtree" housed in
contrib/subtree hierarchy.

* ps/build-meson-subtree:
  meson: wire up the git-subtree(1) command
  meson: introduce build option for contrib
  contrib/subtree: fix building docs

diff --cc meson.build
Simple merge

diff --cc meson_options.txt
Simple merge