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 --combined contrib/subtree/git-subtree.txt
index c5bce41ac7ccd88013f3de3c663f980d2277938c,89614e5bb98cc1122032aabdd2dc54beeda1cba8..7ba853eeda96ec9a4b4a3b4ebb8c88dda055c67d
--- 1/contrib/subtree/git-subtree.txt
--- 2/contrib/subtree/git-subtree.txt
+++ b/contrib/subtree/git-subtree.txt
@@@ -9,7 -9,8 +9,8 @@@ git-subtree - Merge subtrees together a
  SYNOPSIS
  --------
  [verse]
- 'git subtree' add   -P <prefix> <commit>
+ 'git subtree' add   -P <prefix> <refspec>
+ 'git subtree' add   -P <prefix> <repository> <refspec>
  'git subtree' pull  -P <prefix> <repository> <refspec...>
  'git subtree' push  -P <prefix> <repository> <refspec...>
  'git subtree' merge -P <prefix> <commit>
@@@ -93,7 -94,7 +94,7 @@@ pull:
        repository.
        
  push::
 -      Does a 'split' (see above) using the <prefix> supplied
 +      Does a 'split' (see below) using the <prefix> supplied
        and then does a 'git push' to push the result to the 
        repository and refspec. This can be used to push your
        subtree to different branches of the remote repository.