codemod unified-diff generates not applicable patch

Hello,

I've written a codemod which rewrites parameters in given function call. Details are not important. Also, I work in a project where we use Perforce as a source control. As a result I can't use default libcst.tool mode where files are just modified in place as I need to know upfront which files need modification. I've decided to use `-u` option which generates unified diff. I can preprocess such diff to extract files which require modification, open them and the apply the diff. Problem is the generated diff can't be applied when file is "short".

### The problem

Generate diff:
```bash
$ python -m libcst.tool codemod transform_call.ConvertListsToListOfDicts commands/tests/integration_test/file.py -u=3 > /tmp/x.diff
Calculating full-repo metadata...
Executing codemod...
reformatted -

All done! ✨ 🍰 ✨
1 file reformatted.
Finished codemodding 1 files!
- Transformed 1 files successfully.
- Skipped 0 files.
- Failed to codemod 0 files.
- 0 warnings were generated.
```
Apply diff:
```bash
$ patch commands/tests/integration_test/file.py /tmp/x.diff
patching file commands/tests/integration_test/file.py
Hunk #1 FAILED at 3.
1 out of 1 hunk FAILED -- saving rejects to file commands/tests/integration_test/file.py.rej
```

I've compared what is the difference between diff generated by libcst.tool (`x.diff`) an real diff (`y.diff`):
```bash
$ python -m libcst.tool codemod transform_call.ConvertListsToListOfDicts commands/tests/integration_test/file.py
```
```bash
$ diff commands/tests/integration_test/file.py.backup commands/tests/integration_test/file.py -u > /tmp/y.diff
```
```bash
$ diff /tmp/x.diff /tmp/y.diff
1,3c1,3
< --- /home/bill/python_codemods/commands/tests/integration_test/file.py
< +++ /home/bill/python_codemods/commands/tests/integration_test/file.py
< @@ -5,8 +5,15 @@
---
> --- commands/tests/integration_test/file.py.backup 2022-09-27 22:59:06.473918900 +0200
> +++ commands/tests/integration_test/file.py 2022-09-28 01:06:42.863918900 +0200
> @@ -5,7 +5,14 @@
20d19
<
```
It seems that `libcst.tool codemod -u` adds one unneccessary line which prevents `patch` to apply such diff.
```bash
< @@ -5,8 +5,15 @@
---
> @@ -5,7 +5,14 @@
```

If I use `-u=1` to limit context diff is generated correctly and can be applied without any problems. When diff happens to span over entire file (`-u=3`) then it *cannot* be applied.

### Conclusion
Am I using wrong options or diff generation in some cases is wrong?