Procedure for reverting commits on release branch

[Glenn Morris <rgm@gnu.org>]

If a commit is made on the release branch, and then one wants to instead
have it only on master, then given the following commit sequence:

1) emacs-27: do something
2) emacs-27: revert something
3) master: do something

If gitmerge.el is now used to merge emacs-27 to master:
commit 1) is skipped, because it is already on master
commit 2) is merged

Net result: 3) is reverted on master and has to be applied again by hand.

Specific recent example: lisp/calculator.el


A more efficient way to do this would be:
1) emacs-27: do something
2) emacs-27: revert something, and mark "do not merge to master"

Then no special handling is needed.