contrib/subtree: Handle '--prefix' argument with a slash appended

'git subtree merge' will fail if the argument of '--prefix' has a slash
appended.

Signed-off-by: Techlive Zheng <techlivezheng@gmail.com>
Signed-off-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Jeff King <peff@peff.net>

contrib/subtree/git-subtree.sh

@@ -90,7 +90,7 @@ while [ $# -gt 0 ]; do
 		--annotate) annotate="$1"; shift ;;
 		--no-annotate) annotate= ;;
 		-b) branch="$1"; shift ;;
-		-P) prefix="$1"; shift ;;
+		-P) prefix="${1%/}"; shift ;;
 		-m) message="$1"; shift ;;
 		--no-prefix) prefix= ;;
 		--onto) onto="$1"; shift ;;