git-grep: Bail out when -P is used with -F or -E

git-grep: Bail out when -P is used with -F or -E

This patch makes git-grep die() when -P is used on command line together
with -E/--extended-regexp or -F/--fixed-strings.

This also makes it bail out when grep.extendedRegexp is enabled.

But `git grep -G -P pattern` and `git grep -E -G -P pattern` still work
because -G and -E set opts.regflags during parse_options() and there is
no way to detect `-G` or `-E -G`.

Signed-off-by: Michał Kiedrowicz <redacted>
Signed-off-by: Junio C Hamano <redacted>