gitweb: add viewport meta tag for mobile devices

gitweb: add viewport meta tag for mobile devices

Without a viewport meta tag, phone browsers render gitweb at desktop
width and scale the whole page down to fit the screen.

Add a viewport meta tag so the layout viewport tracks device width.
This is the baseline needed for mobile CSS fixes in follow-up commits.

Signed-off-by: Rito Rhymes <rito@ritovision.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>