Don't update the baseline file when the result of the test is identical

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.