When we added the "--perl-regexp" option (or "-P") to "git grep", we
should have done the same for the commands in the "git log" family,
but somehow we forgot to do so.  This corrects it, but we will
reserve the short-and-sweet "-P" option for something else for now.

Also introduce the "--basic-regexp" option for completeness, so that
the "last one wins" principle can be used to defeat an earlier -E
option, e.g. "git log -E --basic-regexp --grep='<bre>'".  Note that
it cannot have the short "-G" option as the option is to grep in the
patch text in the context of "log" family.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>