gitweb: Show project's README.html if available

gitweb: Show project's README.html if available

If the repository includes a README.html file, show it in the summary page.
The usual "this should be in the config file" argument does not apply here
since this can be larger and having such a big string in the config file
would be impractical.

I don't know if this is suitable upstream, but it's one of the repo.or.cz
custom modifications that I've thought could be interesting for others
as well.

Compared to the previous patch, this adds the '.html' extension to the
filename, so that it's clear it is, well, HTML.

Signed-off-by: Petr Baudis <redacted>
Signed-off-by: Junio C Hamano <redacted>