Fix 1529: hashed snippet filename changes should not count as log differences

Fix 1529: hashed snippet filename changes should not count as log differences

If the contents of a regtest change, the regtest gets a new hash and
thus a new filename. Unfortunately, this will be detected by
output-distance as a change and will show up in the regtest analysis.
To work around this, I'm simply extracting the old and the new filename
before we compare the log files and if they differ, I replace the old
filename with the new filename in the old log file. Thus, it will
appear as if the new filename was also used in the baseline and it
will not be flagged as a difference.