bug#37644: html-href-anchor should have TAB expansion when prompting

[積丹尼 Dan Jacobson]

C-c C-c h runs the command html-href-anchor (found in mhtml-mode-map),
which is an interactive compiled Lisp function in ‘sgml-mode.el’.

Problem is it doesn't have filename expansion turned on.

As often we are putting in relative links into a document,
it would be great if we could tab expand even
../../me/bla.html
etc.

Entering http[s] links would not be affected.