rebase-i-exec: Allow space in SHELL_PATH

On Windows, when Git is installed under "C:\Program Files\Git",
SHELL_PATH will include a space. Fix "git rebase --interactive --exec"
so that it works with spaces in SHELL_PATH.

Signed-off-by: Fredrik Medley <fredrik.medley@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

git-rebase--interactive.sh

@@ -610,7 +610,7 @@ do_next () {
 		read -r command rest < "$todo"
 		mark_action_done
 		printf 'Executing: %s\n' "$rest"
-		${SHELL:-@SHELL_PATH@} -c "$rest" # Actual execution
+		"${SHELL:-@SHELL_PATH@}" -c "$rest" # Actual execution
 		status=$?
 		# Run in subshell because require_clean_work_tree can die.
 		dirty=f