tests: add some more actions to test-issue-6028, describe what's being done
This patch does two things: it explains what's happening in the test file so
it's easier to understand, and also it adds checks that make sure something
like swapping merge parents is not accidentally breaking anything.
The primary reason to touch this test file was that it was broken by a change
in core's merge.graft() that erroneously swapped merge parents. Since only
evolve uses merge.graft() for merge commits and there aren't any tests in core
for it, let's test it here. Plus, this test case is pretty simple and these
additional checks don't make it too complicated.