A test in reftable library has been rewritten using the unit test
framework.
* cp/unit-test-reftable-tree:
t-reftable-tree: improve the test for infix_walk()
t-reftable-tree: add test for non-existent key
t-reftable-tree: split test_tree() into two sub-test functions
t: move reftable/tree_test.c to the unit testing framework
reftable: remove unnecessary curly braces in reftable/tree.c