Merge branch 'dg/subtree-fixes'

contrib/subtree updates, but here are only the ones that looked ready. The remainder of the patches will have another day. * dg/subtree-fixes: contrib/subtree: make the manual directory if needed contrib/subtree: honor DESTDIR contrib/subtree: fix synopsis contrib/subtree: better error handling for 'subtree add' contrib/subtree: use %B for split subject/body contrib/subtree: remove test number comments