Hello,

This patch allows preview.el to work in LaTeX-mode non-file buffers
for which TeX-master has been set to the name of a valid tex file.

This patch might appear to be adding a new feature, but if I
understand correctly, it's really fixing an ancient bug.  From what I
can tell, preview.el was carefully designed to support non-file
buffers, but this support was broken unintentionally in the commit
3651776f5a57283f97af54a53a6106519bef41b0.

In more detail, when preview-region is called on a non-file buffer,
the file name "<none>" is passed along to TeX-region-create.  Since
that commit, the file name is subsequently relativized to the master
file, even when it is "<none>".  As a result, in
preview-parse-messages, the checks (string= file "<none>") fail in
cases where they should succeed.  This patch makes it so that
TeX-region-create only relativizes the file name when it is not equal
to "<none>", so that the checks in question succeed, as was intended.

As motivation, I have been using something equivalent to this patch
for a while, and have found it very useful in, e.g., indirect org-mode
source blocks.

Thanks, best,

Paul