install a git-annex-shell shim script when the standalone OSX app runs

I put it in ~/.ssh/ because there's no reliable way to get it into PATH,
and OSX ssh doesn't even honor user's PATH by default.

authorized_keys generators will need to check if it's there. Not done yet.

ui-macos/git-annex.app/Contents/MacOS/runshell

@@ -47,7 +47,7 @@ export GIT_EXEC_PATH
 if [ "$1" ]; then
 	cmd="$1"
 	shift 1
-	"$cmd" "$@"
+	exec "$cmd" "$@"
 else
 	$SHELL
 fi