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* test/lisp/url/url-parse-tests.el: Add tests covering url-generic-parse-url.
* test/lisp/url/url-expand-tests.el: Add tests covering url-expand-file-name.
* lisp/url/url-parse.el (url-generic-parse-url): Keep empty fragment
information in URL-struct.
* lisp/url/url-parse.el (url-path-and-query): Do not artificially turn empty
path and query into nil path and query, respectively.
* lisp/url/url-expand.el (url-expander-remove-relative-links): Do not turn
empty path into an absolute ("/") path.
* lisp/url/url-expand.el (url-expand-file-name): Properly resolve
fragment-only URIs. Do not just return them unchanged.
* lisp/url/url-expand.el (url-default-expander): An empty path in the relative
reference URI should not drop the last segment.

Backport:

(cherry picked from commit b792ecea)