git-p4: replace each tab with 8 spaces for consistency

Note that the majority of git-p4 uses spaces, not tabs, for indentation.
Consistent indentation is a good hygiene for Python scripts, and mixing
tabs and spaces in Python can lead to hard-to-find bugs.

Signed-off-by: Andrew Garber <andrew@andrewgarber.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>