parse-options: add precision handling for PARSE_OPT_CMDMODE

parse-options: add precision handling for PARSE_OPT_CMDMODE

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

Call the function that does the raw casting do_get_int_value() to
reserve the name get_int_value() for a more friendly wrapper we're
going to introduce in one of the next patches.

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