Don't update the baseline file when the result of the test is identical
to the baseline. Since we don't run gzip with the -n option, the output
of gzip varies for identical result files if and when they are created
at different time. Ouch...

Rather than add -n and commit a 600K+ diff for the changes to all the .uu
files, it's less of a churn to uudecode and gunzip the baseline file and
compare that to the new result file to determine if the baseline file
needs to be updated.

This way, "atf-sh mkimg.sh rebase" can be run as many times as people like
and a subsequent "svn status" will not show unnecessary diffs.