decorate: fix sign comparison warnings

decorate: fix sign comparison warnings

There are multiple instances where ints have been initialized with
values of unsigned ints, and where negative values don't mean anything.
When such ints are compared with unsigned ints, it causes sign comparison
warnings.

Also, some of these are used just as stand-ins for their initial
values, never being modified, thus obscuring the specific conditions
under which certain operations happen.

Replace int with unsigned int for 2 variables, and replace the
intermediate variables with their initial values for 2 other variables.

Signed-off-by: Arnav Bhate <bhatearnav@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>