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

Speed up filter-branch for cases where we only care about rewriting
commits, not tree data.

* jk/filter-branch-no-index:
  filter-branch: skip index read/write when possible

t/perf/p7000-filter-branch.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+test_description='performance of filter-branch'
+. ./perf-lib.sh
+
+test_perf_default_repo
+test_checkout_worktree
+
+test_expect_success 'mark bases for tests' '
+	git tag -f tip &&
+	git tag -f base HEAD~100
+'
+
+test_perf 'noop filter' '
+	git checkout --detach tip &&
+	git filter-branch -f base..HEAD
+'
+
+test_done