completion: use os.path.split for zsh script basename

[r3wretrhy]

Bash completion already takes the script basename with os.path.split(sys.argv[0])[1] (#451). The zsh generator still used sys.argv[0].split('/')[-1], so a Windows path such as C:\Python\Scripts\doit.exe is copied whole into the completion script.

Use the same os.path.split as bash.

The added test fails with a literal / split (C: appears in the script) and passes with os.path.split.