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>

contrib/subtree/git-subtree.sh

@@ -497,12 +497,18 @@ cmd_add()
 	ensure_clean
 	
 	if [ $# -eq 1 ]; then
+	    git rev-parse -q --verify "$1^{commit}" >/dev/null ||
+	    die "'$1' does not refer to a commit"
+
 	    "cmd_add_commit" "$@"
 	elif [ $# -eq 2 ]; then
+	    git rev-parse -q --verify "$2^{commit}" >/dev/null ||
+	    die "'$2' does not refer to a commit"
+
 	    "cmd_add_repository" "$@"
 	else
 	    say "error: parameters were '$@'"
-	    die "Provide either a refspec or a repository and refspec."
+	    die "Provide either a commit or a repository and commit."
 	fi
 }