Issue 3675: Make convert-ly -d only ever update on changed files

Issue 3675: Make convert-ly -d only ever update on changed files

Previously, it updated unconditionally whenever a new stable version
came out, leading to merge conflicts.  When the final applied
conversion is to an unstable version and the following stable version
is not beyond the conversion target, the following stable version is
used.

Note that this rule does not make a factual difference for continuous
updates of a code base (the normal use case for
scripts/auxiliar/update-with-convert-ly.sh), but it makes a difference
for the conversion/import of code that may have fallen behind a lot
(like with the LSR import, or when converting archived files).