[PATCH 3/5] reset: use 'skip_cache_tree_update' option

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

From: Victoria Dye <vdye@github.com>

Enable the 'skip_cache_tree_update' option in the variants that call
'prime_cache_tree()' after 'unpack_trees()' (specifically, 'git reset
--mixed' and 'git reset --hard'). This avoids redundantly rebuilding the
cache tree in both 'cache_tree_update()' at the end of 'unpack_trees()' and
in 'prime_cache_tree()', resulting in a small (but consistent) performance
improvement. From the newly-added 'p7102-reset.sh' test:

Test                         before            after
--------------------------------------------------------------------
7102.1: reset --hard (...)   2.11(0.40+1.54)   1.97(0.38+1.47) -6.6%

Signed-off-by: Victoria Dye <vdye@github.com>
---
 builtin/reset.c       |  2 ++
 t/perf/p7102-reset.sh | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100755 t/perf/p7102-reset.sh

diff --git a/builtin/reset.c b/builtin/reset.c
index fdce6f8c856..ab027774824 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -73,9 +73,11 @@ static int reset_index(const char *ref, const struct object_id *oid, int reset_t
 	case HARD:
 		opts.update = 1;
 		opts.reset = UNPACK_RESET_OVERWRITE_UNTRACKED;
+		opts.skip_cache_tree_update = 1;
 		break;
 	case MIXED:
 		opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+		opts.skip_cache_tree_update = 1;
 		/* but opts.update=0, so working tree not updated */
 		break;
 	default:
diff --git a/t/perf/p7102-reset.sh b/t/perf/p7102-reset.sh
new file mode 100755
index 00000000000..9b039e8691f
--- /dev/null
+++ b/t/perf/p7102-reset.sh
@@ -0,0 +1,21 @@
+#!/bin/sh
+
+test_description='performance of reset'
+. ./perf-lib.sh
+
+test_perf_default_repo
+test_checkout_worktree
+
+test_perf 'reset --hard with change in tree' '
+	base=$(git rev-parse HEAD) &&
+	test_commit --no-tag A &&
+	new=$(git rev-parse HEAD) &&
+
+	for i in $(test_seq 10)
+	do
+		git reset --hard $new &&
+		git reset --hard $base || return $?
+	done
+'
+
+test_done
-- 
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