query-replace: Undo replacements performed with 'comma

query-replace: Undo replacements performed with 'comma

During a `query-replace', the char ',' replaces the character
at point and doesn't move point;  right after, the char 'u'
must undo such replacement (Bug#27268).
* lisp/replace.el (replace--push-stack):
New macro extracted from `perform-replace'.
(perform-replace): Use it.
* test/lisp/replace-tests.el (query-replace--undo): Add test.