read-tree: use 'skip_cache_tree_update' option

When running 'read-tree' with a single tree and no prefix,
'prime_cache_tree()' is called after the tree is unpacked. In that
situation, skip a redundant call to 'cache_tree_update()' in
'unpack_trees()' by enabling the 'skip_cache_tree_update' unpack option.

Removing the redundant cache tree update provides a substantial performance
improvement to 'git read-tree <tree-ish>', as shown by a test added to
'p0006-read-tree-checkout.sh':

Test                          before            after
----------------------------------------------------------------------
read-tree br_ballast_plus_1   3.94(1.80+1.57)   3.00(1.14+1.28) -23.9%

Note that the 'read-tree' in 't1022-read-tree-partial-clone.sh' is updated
to read two trees, rather than one. The test was first introduced in
d3da223f22 (cache-tree: prefetch in partial clone read-tree, 2021-07-23) to
exercise the 'cache_tree_update()' code path, as used in 'git merge'. Since
this patch drops the call to 'cache_tree_update()' in single-tree 'git
read-tree', change the test to use the two-tree variant so that
'cache_tree_update()' is called as intended.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/read-tree.c

@@ -249,6 +249,10 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	if (opts.debug_unpack)
 		opts.fn = debug_merge;
 
+	/* If we're going to prime_cache_tree later, skip cache tree update */
+	if (nr_trees == 1 && !opts.prefix)
+		opts.skip_cache_tree_update = 1;
+
 	cache_tree_free(&active_cache_tree);
 	for (i = 0; i < nr_trees; i++) {
 		struct tree *tree = trees[i];

t/perf/p0006-read-tree-checkout.sh

@@ -49,6 +49,14 @@ test_perf "read-tree br_base br_ballast ($nr_files)" '
 	git read-tree -n -m br_base br_ballast
 '
 
+test_perf "read-tree br_ballast_plus_1 ($nr_files)" '
+	# Run read-tree 100 times for clearer performance results & comparisons
+	for i in  $(test_seq 100)
+	do
+		git read-tree -n -m br_ballast_plus_1 || return 1
+	done
+'
+
 test_perf "switch between br_base br_ballast ($nr_files)" '
 	git checkout -q br_base &&
 	git checkout -q br_ballast

t/t1022-read-tree-partial-clone.sh

@@ -19,7 +19,7 @@ test_expect_success 'read-tree in partial clone prefetches in one batch' '
 	git -C server config uploadpack.allowfilter 1 &&
 	git -C server config uploadpack.allowanysha1inwant 1 &&
 	git clone --bare --filter=blob:none "file://$(pwd)/server" client &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client read-tree $TREE &&
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client read-tree $TREE $TREE &&
 
 	# "done" marks the end of negotiation (once per fetch). Expect that
 	# only one fetch occurs.