u_boot_pylib: Clean up pylint warnings in gitutil.py

u_boot_pylib: Clean up pylint warnings in gitutil.py

This file has about 40 pylint warnings, but no errors.

Quite a few of these warnings have been there for a while, but most are
coming from newer versions of pylint, where people come up with new
warnings.

The f-string warning is the most common one:

   C0209: Formatting a regular string which could be an f-string

That feature was not available when the code was written, but it is
often more convenient than using % with a list of arguments.

This patches reduces the number of warnings in this file, with 7 left
remaining.

Signed-off-by: Simon Glass <sjg@chromium.org>