Merge branch 'jk/filter-branch-no-index' into maint

A recent optimization to filter-branch in v2.7.0 introduced a
regression when --prune-empty filter is used, which has been
corrected.

* jk/filter-branch-no-index:
  filter-branch: resolve $commit^{tree} in no-index case

git-filter-branch.sh

@@ -404,7 +404,7 @@ while read commit parents; do
 	then
 		tree=$(git write-tree)
 	else
-		tree="$commit^{tree}"
+		tree=$(git rev-parse "$commit^{tree}")
 	fi
 	workdir=$workdir @SHELL_PATH@ -c "$filter_commit" "git commit-tree" \
 		"$tree" $parentstr < ../message > ../map/$commit ||

t/t7003-filter-branch.sh

@@ -333,6 +333,14 @@ test_expect_success 'prune empty collapsed merges' '
 	test_cmp expect actual
 '
 
+test_expect_success 'prune empty works even without index/tree filters' '
+	git rev-list HEAD >expect &&
+	git commit --allow-empty -m empty &&
+	git filter-branch -f --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 &&