bug#39190: 28.0.50; two buffers with same buffer-file-name (diff-syntax-fontify-props)

[Felician Nemeth]

If I read correctly, diff-syntax-fontify-props sets buffer-file-name of
a temporary buffer to an existing one.  This is not necessarily a bug,
but it definitely looks strange that we have two buffers with different
contents and the same buffer-file-name.

Eglot (in ELPA) runs into problems because of this
(https://github.com/joaotavora/eglot/pull/233).

Is there a recommended way to check in an after-change-major-mode-hook
whether the current buffer is or isn't a temporary diff buffer?  For
example, to check if first character of buffer-name is space.

Thank you.

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo 
version 1.16.0)
 of 2020-01-19 built on betli
Repository revision: 35a1a007bb7506c72ee6d9757a79014c679e7bae
Repository branch: emacs-27
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)