This leverages the same canonicalization btest-diff already applies
during test output comparison against baselines also when updating
those baselines (usually via btest -U/-u). This removes a bunch of
noise from the baselines, including personal home directories,
timestamps, etc.

Since btest-diff doesn't know whether the baseline has undergone this
canonicalization, it continues to canonicalize the basline prior to
diffing. To track whether it has canonicalized a baseline when
updating, btest-diff now also prepends a header to the generated
baseline that warns users about the fact that it is
auto-generated. The presence of this header doubles as a marker for
canonicalization.

Includes a test-case.