fast-import: localize 'i' into the 'for' loops using it

fast-import: localize 'i' into the 'for' loops using it

In cmd_fast_import(), a	local variable 'i' is defined as an
`unsigned int` and then used as a loop counter in four different
`for (i = ...; i < ...; i++) { ... }` loops.

But in three out of the four cases, `unsigned int` isn't the best type
to use.

To give each loop counter the type matching its bound
(int/unsigned/size_t), let's localize 'i' into each loop that uses it.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>