correct `magic %run` completion with arguments

In directory `dir` containing `script.py`, file1`, `file2`, `subdir\file1` and
`subdir\file2`:
`%run script.py <tab>` will give `script.py file1 file2 subdir\`
`%run script.py file<tab>` will give `file1 file2`
`%run script.py file1 <tab>` will give `script.py file1 file2 subdir\`

`%run script.py subdir\file1 <tab>` will give `script.py file1 file2 subdir\`
while before it would have give:
`subdir\file1 script.py file1 file2 subdir\`