t/perf/p3400: speed up setup using fast-import

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. 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 (Average value of 5 tests):
            Real        Rebase
  Before:  29.045s      13.34s
   After:  21.989s      12.84s

Measured on Lenovo Yoga 2020, Ubuntu 24.04.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p3400-rebase.sh

diff --git a/t/perf/p3400-rebase.sh b/t/perf/p3400-rebase.sh
index e6b0277..56dc3e1 100755
--- a/t/perf/p3400-rebase.sh
+++ b/t/perf/p3400-rebase.sh
@@ -9,21 +9,44 @@
 	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 &&
+	sort -nr content_fwd >content_rev &&
+	(
+		for i in $(test_seq 100)
+		do
+			test_tick &&
+			echo "commit refs/heads/upstream" &&
+			echo "committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE" &&
+			echo "data <<EOF" &&
+			echo "commit$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 $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE" &&
+			echo "data <<EOF" &&
+			echo "commit$i-reverse" &&
+			echo "EOF" &&
+			echo "M 100644 inline unrelated-file$i" &&
+			echo "data <<EOF" &&
+			echo "change$i" &&
+			cat content_rev &&
+			echo "EOF" || exit 1
+		done
+	) >fast_import_stream &&
+
+	git fast-import <fast_import_stream &&
+	git repack -a -d &&
+	git checkout -f upstream &&
 	git checkout to-rebase &&
 	test_commit our-patch interesting-file
 '