refs: use ref_namespaces for replace refs base

The git_replace_ref_base global is used to store the value of the
GIT_REPLACE_REF_BASE environment variable or the default of
"refs/replace/". This is initialized within setup_git_env().

The ref_namespaces array is a new centralized location for information
such as the ref namespace used for replace refs. Instead of having this
namespace stored in two places, use the ref_namespaces array instead.

For simplicity, create a local git_replace_ref_base variable wherever
the global was previously used.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

@@ -141,6 +141,7 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
 	enum object_type objtype;
 	enum decoration_type deco_type = DECORATION_NONE;
 	struct decoration_filter *filter = (struct decoration_filter *)cb_data;
+	const char *git_replace_ref_base = ref_namespace[NAMESPACE_REPLACE].ref;
 
 	if (filter && !ref_filter_match(refname, filter))
 		return 0;