Merge branch 'cb/filter-branch-prune-empty-degenerate-merges'

"filter-branch" left an empty single-parent commit that results when
all parents of a merge commit gets mapped to the same commit, even
under "--prune-empty".

* cb/filter-branch-prune-empty-degenerate-merges:
  filter-branch: eliminate duplicate mapped parents

git-filter-branch.sh

@@ -332,7 +332,13 @@ while read commit parents; do
 	parentstr=
 	for parent in $parents; do
 		for reparent in $(map "$parent"); do
-			parentstr="$parentstr -p $reparent"
+			case "$parentstr " in
+			*" -p $reparent "*)
+				;;
+			*)
+				parentstr="$parentstr -p $reparent"
+				;;
+			esac
 		done
 	done
 	if [ "$filter_parent" ]; then

t/t7003-filter-branch.sh

@@ -308,6 +308,17 @@ test_expect_success 'Prune empty commits' '
 	test_cmp expect actual
 '
 
+test_expect_success 'prune empty collapsed merges' '
+	test_config merge.ff false &&
+	git rev-list HEAD >expect &&
+	test_commit to_remove_2 &&
+	git reset --hard HEAD^ &&
+	test_merge non-ff to_remove_2 &&
+	git filter-branch -f --index-filter "git update-index --remove to_remove_2.t" --prune-empty HEAD &&
+	git rev-list HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '--remap-to-ancestor with filename filters' '
 	git checkout master &&
 	git reset --hard A &&