grep API: plug memory leaks by freeing "header_list"

grep API: plug memory leaks by freeing "header_list"

When the "header_list" struct member was added in [1], freeing this
field was neglected. Fix that now, so that commands like

	./git -P log -1 --color=always --author=A origin/master

will run leak-free.

1. 80235ba79ef ("log --author=me --grep=it" should find intersection,
   not union, 2010-01-17)

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