cache-tree: implement cache_tree_find_path()

cache-tree: implement cache_tree_find_path()

Given a 'struct cache_tree', it may be beneficial to navigate directly
to a node within that corresponds to a given path name. Create
cache_tree_find_path() for this function. It returns NULL when no such
path exists.

The implementation is adapted from do_invalidate_path() which does a
similar search but also modifies the nodes it finds along the way. The
method could be implemented simply using tail-recursion, but this while
loop does the same thing.

This new method is not currently used, but will be in an upcoming
change.

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