Merge branch 'ns/merge-base-is-ancestor-tests' into jch

Tests for 'git merge-base --is-ancestor' have been added to cover
exit codes (0 for success, 1 for non-ancestor, 128 for errors) and
to ensure it cannot be combined with '--all'.

* ns/merge-base-is-ancestor-tests:
  merge-base: add tests for --is-ancestor