pickaxe: die when -G and --pickaxe-regex are combined

pickaxe: die when -G and --pickaxe-regex are combined

When the -G and --pickaxe-regex options are combined we simply ignore
the --pickaxe-regex option. Let's die instead as suggested by our
documentation, since -G is always a regex.

When --pickaxe-regex was added in d01d8c6782 (Support for pickaxe
matching regular expressions, 2006-03-29) only the -S option
existed. Then when -G was added in f506b8e8b5 (git log/diff: add
-G<regexp> that greps in the patch text, 2010-08-23) neither the
documentation for --pickaxe-regex was updated accordingly, nor was
something like this assertion added.

Since 5bc3f0b567 (diffcore-pickaxe doc: document -S and -G properly,
2013-05-31) we've claimed that --pickaxe-regex should only be used
with -S, but have silently tolerated combining it with -G, let's die
instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>