`copy_helper_opts.read_args_from_stdin` code means that `cat retrieve-files | gsutil cp -I . -L log` != `cat retrieve-files | gsutil cp -L log -I .`

```
-I Use ``stdin`` to specify a list of files or objects to copy. You can use
gsutil in a pipeline to upload or download objects as generated by a program.
For example:

cat filelist | gsutil -m cp -I gs://my-bucket

where the output of ``cat filelist`` is a one-per-line list of
files, cloud URLs, and wildcards of files and cloud URLs.
```

```
-L Outputs a manifest log file with detailed information about
each item that was copied. This manifest contains the following
information for each item:

- Source path.
- Destination path.
- Source size.
- Bytes transferred.
- MD5 hash.
- Transfer start time and date in UTC and ISO 8601 format.
- Transfer completion time and date in UTC and ISO 8601 format.
- Upload id, if a resumable upload was performed.
- Final result of the attempted transfer, either success or failure.
- Failure details, if any.

If the log file already exists, gsutil uses the file as an
input to the copy process, and appends log items to
the existing file. Objects that are marked in the
existing log file as having been successfully copied or
skipped are ignored. Objects without entries are
copied and ones previously marked as unsuccessful are
retried. This option can be used in conjunction with the ``-c`` option to
build a script that copies a large number of objects reliably,
using a bash script like the following:

until gsutil cp -c -L cp.log -r ./dir gs://bucket; do
sleep 1
done

The -c option enables copying to continue after failures
occur, and the -L option allows gsutil to pick up where it
left off without duplicating work. The loop continues
running as long as gsutil exits with a non-zero status. A non-zero
status indicates there was at least one failure during the copy
operation.

NOTE: If you are synchronizing the contents of a
directory and a bucket, or the contents of two buckets, see
"gsutil help rsync".
```
Nothing here _appears_ to say _you must place `-L ...` before `-I ...`_, but the condition below forces it:

https://github.com/GoogleCloudPlatform/gsutil/blob/a32d8f5596c256c0de3ac60810278bd68738751f/gslib/commands/cp.py#L1066-L1068

When run in one order, things work fine, in the other, one gets:
> CommandException: Source URLs cannot be specified with -I option

Start at gslib/commands/cp.py around lines 1066-1068 and trace how copy_helper_opts.read_args_from_stdin handles -I and -L. Reproduce both command orders with the reported pipeline; done means both orders are accepted consistently without the “Source URLs cannot be specified with -I option” error.

Written by the indexing model from the issue text.