Merge branch 'fm/shell-path-whitespace' into maint

Portability fix for Windows, which may rewrite $SHELL variable using
non-POSIX paths.

* fm/shell-path-whitespace:
  rebase-i-exec: Allow space in SHELL_PATH

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