git-compat-util: convert string predicates to return bool

git-compat-util: convert string predicates to return bool

Since 8277dbe987 (git-compat-util: convert skip_{prefix,suffix}{,_mem}
to bool, 2023-12-16) a number of our string predicates have been
returning bool instead of int. Now that we've declared that experiment
a success, let's convert the return type of the case-independent
skip_iprefix() and skip_iprefix_mem() functions to match the return
type of their case-dependent equivalents. Returning bool instead of
int makes it clear that these functions are predicates.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>