read-cache: use size_t for {base,df}_name_compare()

read-cache: use size_t for {base,df}_name_compare()

Support names of any length in base_name_compare() and df_name_compare()
by using size_t for their length parameters.  They pass the length on to
memcmp(3), which also takes it as a size_t.

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