gitweb: Add support for grep searches

gitweb: Add support for grep searches

The 'grep' type of search greps the currently selected tree for given
regexp and shows the results in a fancy table with links into blob view.
The number of shown matches is limited to 1000 and the whole feature
can be turned off (grepping linux-2.6.git already makes repo.or.cz a bit
unhappy).

This second revision makes it in documentation explicit that grep accepts
regexps, and makes grep accept extended regexps instead of basic regexps.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>