In go 1.22, loop variables are redeclared with each iteration of the
loop, rather than simple updated on each iteration. This means that we
no longer need to manually redeclare variables when they're closed over
by a function.
Changing globals in the init() function of a test file is a bad idea, as it
affects all other tests that run after it. Do it explicitly in each test
function that needs it, and take care of restoring the previous value
afterwards.
The root commit is special in that it has no parents. So we need to add a pipe that's headed for a commit
that doesn't actually exist i.e. the mythical empty tree commit. We're using the actual hash of that
pseudo-commit, but it's not being read anywhere.
