filter-branch: subdirectory filter needs --full-history

filter-branch: subdirectory filter needs --full-history

When two branches are merged that modify a subdirectory (possibly in
different intermediate steps) such that both end up identical, then
rev-list chooses only one branch. But when we filter history, we want to
keep both branches. Therefore, we must use --full-history.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>