gitweb: Clearly distinguish regexp / exact match searches

gitweb: Clearly distinguish regexp / exact match searches

This patch does a couple of things:

* Makes commit/author/committer search case insensitive

  To be consistent with the grep search; I see no convincing
  reason for the search to be case sensitive, and you might
  get in trouble especially with contributors e.g. from Japan
  or France where they sometimes like to uppercase their last
  name.

* Makes grep search by default search for fixed strings.

* Introduces 're' checkbox that enables POSIX extended regexp searches

  This works for all the search types. The idea comes from Jakub.

It does not make much sense (and is not easy at all) to untangle most
of these changes from each other, thus they all go in a single patch.

[jn: Cherry-picked from Pasky's http://repo.or.cz/git/gitweb.git]

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>