[PATCH v2] t/perf/p3400: speed up setup using fast-import

[Tian Yuchen <a3205153416@gmail.com>]

The setup phase in 't/perf/p3400-rebase.sh' generates 100 commits to
simulate a noisy history. It currently uses a shell loop that invokes
'git add', 'git commit', 'test_seq', and 'sort' in each iteration.
This incurs significant overhead due to repeated process spawning.

Optimize the setup by using 'git fast-import' to generate the commit
history in a single stream. Additionally, pre-compute the forward and
reversed file contents to avoid repetitive execution of 'seq' and 'sort'.

To ensure the test measures rebase performance against a consistent
object layout (rather than the suboptimal pack/loose objects created
by the raw import), perform a full repack (`git repack -a -d`) at the
end of the setup.

This reduces the setup time significantly while maintaining the validity
of the subsequent performance tests.

Performance enhancement:
          Real         Rebase
  Before: 29.045s      13.34s
  After:  22.231s      12.78s

Measured on Lenovo Yoga 2020, Ubuntu 24.04.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
---

Changes since v1:
- Added 'git repack -a -d' at the end of the setup phase.
- This fixes a performance regression observed in the subsequent rebase
  test, ensuring the object layout is normalized before testing.
- Updated setup timing in the commit message to reflect the repack overhead.

 t/perf/p3400-rebase.sh | 54 +++++++++++++++++++++++++++++-------------
 1 file changed, 38 insertions(+), 16 deletions(-)

diff --git a/t/perf/p3400-rebase.sh b/t/perf/p3400-rebase.sh
index e6b0277729..9f4251aed6 100755
--- a/t/perf/p3400-rebase.sh
+++ b/t/perf/p3400-rebase.sh
@@ -9,25 +9,47 @@ test_expect_success 'setup rebasing on top of a lot of changes' '
 	git checkout -f -B base &&
 	git checkout -B to-rebase &&
 	git checkout -B upstream &&
-	for i in $(test_seq 100)
-	do
-		# simulate huge diffs
-		echo change$i >unrelated-file$i &&
-		test_seq 1000 >>unrelated-file$i &&
-		git add unrelated-file$i &&
-		test_tick &&
-		git commit -m commit$i unrelated-file$i &&
-		echo change$i >unrelated-file$i &&
-		test_seq 1000 | sort -nr >>unrelated-file$i &&
-		git add unrelated-file$i &&
-		test_tick &&
-		git commit -m commit$i-reverse unrelated-file$i ||
-		return 1
-	done &&
+
+	test_seq 1000 >content_fwd &&
+	test_seq 1000 | sort -nr >content_rev &&
+
+	(
+		for i in $(test_seq 100)
+		do
+			echo "commit refs/heads/upstream" &&
+			echo "committer WGYDY <author@mock.com> $i +0000" &&
+			echo "data <<EOF" &&
+			echo "commit$i" &&
+			echo "EOF" &&
+			
+			if test "$i" = 1; then
+				echo "from refs/heads/upstream^0"
+			fi &&
+
+			echo "M 100644 inline unrelated-file$i" &&
+			echo "data <<EOF" &&
+			echo "change$i" &&
+			cat content_fwd &&
+			echo "EOF" &&
+
+			echo "commit refs/heads/upstream" &&
+			echo "committer WGYDY <author@mock.com> $i +0000" &&
+			echo "data <<EOF" &&
+			echo "commit$i-reversed" &&
+			echo "EOF" &&
+			echo "M 100644 inline unrelated-file$i" &&
+			echo "data <<EOF" &&
+			echo "change$i" &&
+			cat content_rev &&
+			echo "EOF" || return 1
+		done
+	) | git fast-import &&
+
+	git repack -a -d &&
+	git checkout -f upstream &&
 	git checkout to-rebase &&
 	test_commit our-patch interesting-file
 '
-
 test_perf 'rebase on top of a lot of unrelated changes' '
 	git rebase --onto upstream HEAD^ &&
 	git rebase --onto base HEAD^
-- 
2.43.0