cache,tree: move cmp_cache_name_compare from tree.[ch] to read-cache.c

cache,tree: move cmp_cache_name_compare from tree.[ch] to read-cache.c

Since cmp_cache_name_compare() was comparing cache_entry structs, it
was associated with the cache rather than with trees.  Move the
function.  As a side effect, we can make cache_name_stage_compare()
static as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>