gitweb: Fix searchbox positioning

gitweb: Fix searchbox positioning

Currently, searchbox is CSS'd to have position: absolute, which has the
unfortunate consequence that if the viewport is too small and can't fit
into the page width together with the navbar, it gets overlapped and part
of the navbar gets obscured. This makes searchbox float: right instead,
thus the navbar simply gets wrapped.

Discovered and fix pointed out by Michael Olson <redacted>.

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