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>

git-filter-branch.sh

@@ -232,11 +232,11 @@ mkdir ../map || die "Could not create map/ directory"
 case "$filter_subdir" in
 "")
 	git rev-list --reverse --topo-order --default HEAD \
-		--parents "$@"
+		--parents --simplify-merges "$@"
 	;;
 *)
 	git rev-list --reverse --topo-order --default HEAD \
-		--parents "$@" -- "$filter_subdir"
+		--parents --simplify-merges "$@" -- "$filter_subdir"
 esac > ../revs || die "Could not get the commits"
 commits=$(wc -l <../revs | tr -d " ")
 
@@ -326,7 +326,8 @@ then
 	do
 		sha1=$(git rev-parse "$ref"^0)
 		test -f "$workdir"/../map/$sha1 && continue
-		ancestor=$(git rev-list -1 $ref -- "$filter_subdir")
+		ancestor=$(git rev-list --simplify-merges -1 \
+				$ref -- "$filter_subdir")
 		test "$ancestor" && echo $(map $ancestor) >> "$workdir"/../map/$sha1
 	done < "$tempdir"/heads
 fi