gitweb: Separate search regexp from search text

gitweb: Separate search regexp from search text

Separate search text, which is saved in $searchtext global variable,
and is used in links, as default value for the textfield in search
form, and for pickaxe search, from search regexp, which is saved in
$search_regexp global variable, and is used as parameter to --grep,
--committer or --author options to git-rev-list, and for searching
commit body in gitweb.  For now $search_regexp is unconditionallt
equal to quotemeta($searchtext), meaning that we always search for
fixed string.

This fixes bug where 'next page' links for 'search' view didn't work
for searchtext containing quotable characters, like `@'.

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