Merge branch 'cp/unit-test-reftable-tree'

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