Use fixed-size integers when writing out the index in fast-import.

Use fixed-size integers when writing out the index in fast-import.

Currently the pack .idx file format uses 32-bit unsigned integers
for the fan-out table and the object offsets.  We had previously
defined these as 'unsigned int', but not every system will define
that type to be a 32 bit value.  To ensure maximum portability we
should always use 'uint32_t'.

Signed-off-by: Shawn O. Pearce <redacted>