bug#11839: teach some url code about .xz files

[Glenn Morris]

Liam Stitt wrote:

> Hi.  The code in lisp/url knows about .gz and .bz2 files, but not about the
> more recent .xz variety.  These two trivial patches correct this:

Thanks. This seems reasonable, but I have no idea what this code is for
(url-file-find-possibly-compressed-file etc), so I cannot test this.
Can you give an example of how it can be used?

Also, doesn't this line of url-file:

(setq uncompressed-filename
   (if (string-match "\\.\\(gz\\|Z\\|z\\)$"  filename)
                    (substring filename 0 (match-beginning 0)) filename))
need bz2 etc adding?

(Also I feel like lists of compressed file extensions are being
replicated in several places in Emacs: here in url-file,
dired-compress-file-suffixes, jka-compr-compression-info-list,
ibuffer-compressed-file-name-regexp, tags-compression-info-list, ...)