Support for undo-amalgamate in a version of the atomic-change-group macro (with patch)

[Campbell Barton]

Hi, there have been a handful of times I've needed to treat multiple
actions as a single action,
others too (see stackoverflow question [0]).

While this is supported using change groups, the resulting macro ends
up being nearly identical to the existing `atomic-change-group' macro.
The only difference is a call to undo-amalgamate-change-group.

Attached a patch to add `atomic-change-group-undo-amalgamate' so
packages don't need to use a modified copy of this macro.

[0]: https://emacs.stackexchange.com/questions/7558/how-to-collapse-undo-history

-- 
- Campbell

emacs-atomic-change-group-undo-amalgamate.diff
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