cat-file: declare loop counter inside for()

cat-file: declare loop counter inside for()

Declare loop counters in the for statement when they are only used
within the loop body, limiting their scope and improving readability.

While updating the loop counters, use size_t instead of int for counters
that iterate over object counts.

Update the 'nr' parameter of dispatch_calls() to size_t as all callers
already pass a value of that type.

Helped-by: Christian Couder <redacted>
Signed-off-by: Eric Ju <redacted>
Signed-off-by: Pablo Sabater <redacted>
Signed-off-by: Junio C Hamano <redacted>