filter-branch: use --simplify-merges

filter-branch: use --simplify-merges

Use rev-list --simplify-merges everywhere.  This changes the behaviour
of --subdirectory-filter in cases such as

  O -- A -\
   \       \
    \- B -- M

where A and B bring the same changes to the subdirectory: It now keeps
both sides of the merge.  Previously, the history would have been
simplified to 'O -- A'.  Merges of unrelated side histories that never
touch the subdirectory are still removed.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>