Remote file: first heading is replaced with GitHub page HTML #166

Steps

./gh-md-toc https://github.com/ekalinin/github-markdown-toc/blob/master/README.md

Expected

* [gh-md-toc](#gh-md-toc)
* [Table of contents](#table-of-contents)
...

Actual

The first entry contains GitHub page markup instead of the heading:

   * [<button data-component="Button" type="button" aria-label="Expand file tree" ...
* [Table of contents](#table-of-contents)
...

The line is about 30 KB long, its href points to the repository tree URL
(/<owner>/<repo>/tree/<sha>) instead of the heading anchor, and it is indented
as a second-level entry. The other entries are correct.

Reproduced with gh-md-toc 0.10.0. The same happens for
https://github.com/ekalinin/sitemap.js/blob/6bc3eb12c898c1037a35a11b2eb24ababdeb3580/README.md
(the URL from the commented-out "TOC for remote README.md" test).
Wiki pages, e.g. https://github.com/ekalinin/nodeenv/wiki/Who-Uses-Nodeenv, are not affected.