bug#21684: 25.0.50; undo in query-replace w/o exit

[Tino Calancha]

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
 of 2015-10-14
Repository revision: 38f99a02b83e9e408970cb7abdb685725b2004f9


Quite often, performing query-replace, i just hit the [space] quicky
with no much attention. If i replace one string by mistake, i can
comeback to it with "^", but if i edit the string with "E" to reset it 
to its original value, the interactive query-replace session ends.

I found useful if we can undo such replacements without exiting the 
query-replace.

The attached patch implement such behaviour for `query-replace'.
Having this also working for `query-replace-regexp' would be very nice,
but harder to implement.

replace.patch
Description: Text document