tree-walk.c: break circular dependency with unpack-trees

The unpack-trees API depends on the tree-walk API. But we've recently
introduced a dependency in tree-walk.c on MAX_UNPACK_TREES, which
doesn't otherwise care about unpack-trees at all.

Let's break that dependency by reversing the constants: we'll introduce
a new MAX_TRAVERSE_TREES which belongs to the tree-walk API. And then we
can define MAX_UNPACK_TREES in terms of that (since unpack-trees cannot
possibly work with more trees than it can traverse at once via
tree-walk).

The value for both will remain at 8. This is somewhat arbitrary and
probably more than is necessary, per ca885a4fe6 (read-tree() and
unpack_trees(): use consistent limit, 2008-03-13), but there's not
really any pressing need to reduce it.

Suggested-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.h

@@ -6,7 +6,7 @@
 #include "string-list.h"
 #include "tree-walk.h"
 
-#define MAX_UNPACK_TREES 8
+#define MAX_UNPACK_TREES MAX_TRAVERSE_TREES
 
 struct cache_entry;
 struct unpack_trees_options;