web--browse: better support for chromium

web--browse: better support for chromium

On Debian-based distributions, Chromium the browser is available under
the name chromium-browser rather than chromium, to prevent conflicts
with the Chromium B.S.U. game.

Look for chromium-browser first when setting the path for chromium, and
also add chromium-browser as a supported browser name. Document the
dual-name support, and mention the dual-name support for
(google-)chrome too.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>