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

diff --cc contrib/subtree/git-subtree.txt
Simple merge