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

meson.build

@@ -1879,7 +1879,6 @@ endforeach
 if intl.found()
   subdir('po')
 endif
-subdir('contrib')
 
 # Gitweb requires Perl, so we disable the auto-feature if Perl was not found.
 # We make sure further up that Perl is required in case the gitweb option is
@@ -1906,6 +1905,8 @@ if get_option('docs') != []
   subdir('Documentation')
 endif
 
+subdir('contrib')
+
 foreach key, value : {
   'DIFF': diff.full_path(),
   'GIT_TEST_CMP': diff.full_path() + ' -u',