bug#44614: 26.3; bibtex convert case undefined

[Francesco Potortì]

GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of 
2020-05-17, modified by Debian

bibtex.el offers the opportunity of unifying the case of fields (see the
string unify-case).  Unfortunately the docs and comments do not tell
what is the chosen case.  In practice, I see that entries are downcased.

An old version of bibtex.el (last copyright 1997) uniquified cases based
on the bibtex-unify-case-convert variable, which is now removed.

This is unfortunate, and in my opinion a bug.

Looking at the old code, I see that the current behaviour is the same as
the old 'identity value for the mentioned variable.  I am working on a
local patch for reverting to the old behaviour.  After I finish, I can
provide it, if there is any hope of it to be accepted.

Here is the old definition:

(defcustom bibtex-unify-case-convert 'downcase
  "*Function called when unifying case on entry and field names.
This variable is buffer-local."
  :group 'bibtex
  :type '(choice (const :tag "Same case as in `bibtex-field-alist'" identity)
                 (const :tag "Downcase" downcase)
                 (const :tag "Capitalize" capitalize)
                 (const :tag "Upcase" upcase)
                 (function :tag "Conversion function")))
(make-variable-buffer-local 'bibtex-unify-case-convert)