t5538: add test to confirm deltas in shallow pushes

It can be notoriously difficult to detect if delta bases are being
computed properly during 'git push'. Construct an example where it will
make a kilobyte worth of difference when a delta base is not found. We
can then use the progress indicators to distinguish between bytes and
KiB depending on whether the delta base is found and used.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

Diff for: t/t5538-push-shallow.sh

@@ -136,4 +136,25 @@ test_expect_success 'push new commit from shallow clone has correct object count
 	test_grep "Enumerating objects: 1, done." err
 '
 
+test_expect_success 'push new commit from shallow clone has good deltas' '
+	git init base &&
+	test_seq 1 999 >base/a &&
+	test_commit -C base initial &&
+	git -C base add a &&
+	git -C base commit -m "big a" &&
+
+	git clone --depth=1 "file://$(pwd)/base" deltas &&
+	git -C deltas checkout -b deltas &&
+	test_seq 1 1000 >deltas/a &&
+	git -C deltas commit -a -m "bigger a" &&
+	GIT_TRACE2_PERF="$(pwd)/trace.txt" \
+	GIT_PROGRESS_DELAY=0 git -C deltas push --progress origin deltas 2>err &&
+
+	test_grep "Enumerating objects: 5, done" err &&
+
+	# If the delta base is found, then this message uses "bytes".
+	# If the delta base is not found, then this message uses "KiB".
+	test_grep "Writing objects: .* bytes" err
+'
+
 test_done