contrib/subtree: better error handling for 'subtree add'

Check refspecs for validity before passing them on to other commands.
This lets us generate more helpful error messages.

Signed-off-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 5598210..771f39d 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
Binary files differ