Bug (5.36): ` gsutil cp -I` copies only the first two files

From gsutil 5.35 to 5.36, the undocumented regression in the title appeared.

### Steps to reproduce the issue:
1. Download versions 5.35 and 5.36.

```sh
rm -rf /tmp/gsutil535 /tmp/gsutil536
mkdir -p /tmp/gsutil535 /tmp/gsutil536

curl -fsSL https://gcr.io/v2/cloud-builders/gsutil/blobs/sha256:153ea87877c8f60cf9a00ccfef2ba097cdfcd815e1a12c1e8f5e98e999f2a4d0 | tar -xzf - -C /tmp/gsutil535 builder/google-cloud-sdk
curl -fsSL https://gcr.io/v2/cloud-builders/gsutil/blobs/sha256:766e040858e15ade8471ba94a9eb0b7099d2fc01aa92cff2fac76163b8dd0cf3 | tar -xzf - -C /tmp/gsutil536 builder/google-cloud-sdk

CLOUDSDK_PYTHON=python3 /tmp/gsutil535/builder/google-cloud-sdk/bin/gsutil version -l
```

It will print gsutil version: 5.35

```
CLOUDSDK_PYTHON=python3 /tmp/gsutil536/builder/google-cloud-sdk/bin/gsutil version -l
```

It will print gsutil version: 5.36

———

2. Test version 5.35.

```
rm -rf /tmp/gsutil-535-google4
mkdir -p /tmp/gsutil-535-google4

cat <<'EOF' | sed '/^$/d' | CLOUDSDK_PYTHON=python3 /tmp/gsutil535/builder/google-cloud-sdk/bin/gsutil -q -m cp -I /tmp/gsutil-535-google4
gs://cloud-samples-data/language/entity.txt
gs://cloud-samples-data/language/entity-sentiment.txt
gs://cloud-samples-data/language/syntax-sentence.txt
gs://cloud-samples-data/language/classify-entertainment.txt
EOF

ls -1 /tmp/gsutil-535-google4 | sort
```

It will correctly print:

```
classify-entertainment.txt
entity-sentiment.txt
entity.txt
syntax-sentence.txt
```

———

3. Test version 5.36.

```
rm -rf /tmp/gsutil-536-google4
mkdir -p /tmp/gsutil-536-google4

cat <<'EOF' | sed '/^$/d' | CLOUDSDK_PYTHON=python3 /tmp/gsutil536/builder/google-cloud-sdk/bin/gsutil -q -m cp -I /tmp/gsutil-536-google4
gs://cloud-samples-data/language/entity.txt
gs://cloud-samples-data/language/entity-sentiment.txt
gs://cloud-samples-data/language/syntax-sentence.txt
gs://cloud-samples-data/language/classify-entertainment.txt
EOF

ls -1 /tmp/gsutil-536-google4 | sort
```

Bug: it will print only the first two files, showing it did not download all four files.

```
entity-sentiment.txt
entity.txt
```