bug#34027: 27.0.50; disable-theme resets variables to their initial values

[Michael Albinus]

Scenario:

- Declare a variable (either defvar or defcustom)
- Change the initial value to something else
- Enable a theme, which changes the value to something different, again
- Disable the theme

I would expect that the variable has been reset to the value prior
enabling the theme. But it is reset to the initial value.

The documentation of disable-theme isn't helpful, it just says "Disable
all variable and face settings defined by THEME."

I've added the test case custom--test-theme-variables in
test/lisp/custom-resources/custom--test-theme.el for showing the effect.


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.1)
 of 2018-12-27 built on detlef
Repository revision: 13dfe15ef4bc63ce66daffdc502a92aa9be44d92
Repository branch: master
System Description: Ubuntu 18.10

Recent messages:
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nnml: Reading incoming mail (no new mail)...done
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