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>

tree.h

@@ -29,7 +29,6 @@ void free_tree_buffer(struct tree *tree);
 /* Parses and returns the tree in the given ent, chasing tags and commits. */
 struct tree *parse_tree_indirect(const struct object_id *oid);
 
-int cmp_cache_name_compare(const void *a_, const void *b_);
 
 #define READ_TREE_RECURSIVE 1
 typedef int (*read_tree_fn_t)(const struct object_id *, struct strbuf *, const char *, unsigned int, void *);