ref-cache: use 'size_t' instead of int for length

ref-cache: use 'size_t' instead of int for length

The commit 090eb5336c (refs: selectively set prefix in the seek
functions, 2025-07-15) modified the ref-cache iterator to support
seeking to a specified marker without setting the prefix.

The commit adds and uses an integer 'len' to capture the length of the
seek marker to compare with the entries of a given directory. Since the
type of the variable is 'int', this is met with a typecast of converting
a `strlen` to 'int' so it can be assigned to the 'len' variable.

This is whole operation is a bit wrong:
1. Since the 'len' variable is eventually used in a 'strncmp', it should
have been of type 'size_t'.
2. This also truncates the value provided from 'strlen' to an int, which
could cause a large refname to produce a negative number.

Let's do the correct thing here and simply use 'size_t' for `len`.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>