graph: make the ahead-behind docs clearer

Explain it in local-upstream branch terms so it's easier to grasp than
with the `one` and `two` naming from the merge-base code.
Update to latest Clar
Update cl_assert_equal_sz to be nicer

This makes the size_t comparison test nicer (assuming that the
values are actually not using the full length), and converts
some cases that were using it for pointer comparison to use the
macro that is designed for pointer comparison.
Add cl_repo_set_bool and cleanup tests

This adds a helper function for the cases where you want to
quickly set a single boolean config value for a repository.
This allowed me to remove a lot of code.
Merge pull request #1432 from arrbee/update-clar

Update clar and some test helpers clean up