filter-branch: skip index read/write when possible

If the user specifies an index filter but not a tree filter,
filter-branch cleverly avoids checking out the tree
entirely. But we don't do the next level of optimization: if
you have no index or tree filter, we do not need to read the
index at all.

This can greatly speed up cases where we are only changing
the commit objects (e.g., cementing a graft into place).
Here are numbers from the newly-added perf test:

  Test                  HEAD^              HEAD
  ---------------------------------------------------------------
  7000.2: noop filter   13.81(4.95+0.83)   5.43(0.42+0.43) -60.7%

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index 27c9c54..d61f9ba 100755
--- a/git-filter-branch.sh
+++ b/git-filter-branch.sh
@@ -306,6 +306,15 @@
 	start_timestamp=$(date '+%s')
 fi
 
+if test -n "$filter_index" ||
+   test -n "$filter_tree" ||
+   test -n "$filter_subdir"
+then
+	need_index=t
+else
+	need_index=
+fi
+
 while read commit parents; do
 	git_filter_branch__commit_count=$(($git_filter_branch__commit_count+1))
 
@@ -313,7 +322,10 @@
 
 	case "$filter_subdir" in
 	"")
-		GIT_ALLOW_NULL_SHA1=1 git read-tree -i -m $commit
+		if test -n "$need_index"
+		then
+			GIT_ALLOW_NULL_SHA1=1 git read-tree -i -m $commit
+		fi
 		;;
 	*)
 		# The commit may not have the subdirectory at all
@@ -387,8 +399,15 @@
 	} <../commit |
 		eval "$filter_msg" > ../message ||
 			die "msg filter failed: $filter_msg"
+
+	if test -n "$need_index"
+	then
+		tree=$(git write-tree)
+	else
+		tree="$commit^{tree}"
+	fi
 	workdir=$workdir @SHELL_PATH@ -c "$filter_commit" "git commit-tree" \
-		$(git write-tree) $parentstr < ../message > ../map/$commit ||
+		"$tree" $parentstr < ../message > ../map/$commit ||
 			die "could not write rewritten commit"
 done <../revs
 
diff --git a/t/perf/p7000-filter-branch.sh b/t/perf/p7000-filter-branch.sh
new file mode 100755
index 0000000..15ee5d1
--- /dev/null
+++ b/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