git-grep: do not die upon -F/-P when grep.extendedRegexp is set.

git-grep: do not die upon -F/-P when grep.extendedRegexp is set.

The previous one made "git grep -P" fail when grep.extendedRegexp is
enabled.  That is a no-starter.  The option on the command line should
just make the command ignore the configured default.  The handling of "-F"
in the existing code has the same problem.

Instead of saying -G/-F/-E/-P incompatible with each other, just allow
the last one win.  That way, you can have "[alias] gr = grep -P" and
use Pcre for everyday work e.g. "git gr ':i?foo'", and append -G to the
aliased command line to override it e.g. "git gr -G '[Ff][Oo][Oo]'".

Signed-off-by: Junio C Hamano <gitster@pobox.com>