git-p4: removed brackets when assigning multiple return values

In several places, git-p4 contains code of the form:

(a, b) = foo()

In each case, multiple values are returned through a tuple or a list and
bound into multiple values.

The brackets around the assigned variables are redundant and can be
removed:

a, b = foo()

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
