gitweb: Serve text/* 'blob_plain' as text/plain with $prevent_xss

gitweb: Serve text/* 'blob_plain' as text/plain with $prevent_xss

One of mechanism enabled by setting $prevent_xss to true is 'blob_plain'
view protection.  With XSS prevention on, blobs of all types except a
few known safe ones are served with "Content-Disposition: attachment" to
make sure they don't run in our security domain.

Instead of serving text/* type files, except text/plain (and including
text/html), as attachements, downgrade it to text/plain.  This way HTML
pages in 'blob_plain' (raw) view would be displayed in browser, but
safely as a source, and not asked to be saved.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>