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

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