parse-options: add precision handling for OPTION_SET_INT

parse-options: add precision handling for OPTION_SET_INT

Similar to 09705696f7 (parse-options: introduce precision handling for
`OPTION_INTEGER`, 2025-04-17) support value variables of different sizes
for OPTION_SET_INT.  Do that by requiring their "precision" to be set,
casting their "value" pointer accordingly and checking whether the value
fits.

Factor out the casting code from the part of do_get_value() that handles
OPTION_INTEGER to avoid code duplication.  We're going to use it in the
next patches as well.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>