Make convert-ly -d only ever update on changed files (issue 31830043)

[k-ohara5a5a]

I'm fine with this,
but then we would probably want to run convert-ly without the '-d'
option on Documentation/snippets, upon creation of version 2.18.0.

Maybe there is a way to get the behavior of "round-up-to stable version"
triggered by the (usually dummy) convert-ly rule for that version.  I
can only think of kludgy ways right now


https://codereview.appspot.com/31830043/diff/1/scripts/convert-ly.py
File scripts/convert-ly.py (left):

https://codereview.appspot.com/31830043/diff/1/scripts/convert-ly.py#oldcode295
scripts/convert-ly.py:295: # check the y in x.y.z  (minor version
number)
This did cause me confusion recently, while preparing the keySignature
name-change that I had postponed for the 2.19.x branch.

I knew about the mechanism to update to a stable label using a
dummy-rule mechanism in convertrules.py, but this hidden mechanism
surprised me.  Why, I thought, does it know about 2.18.0 when that
version does not exist?

https://codereview.appspot.com/31830043/diff/1/scripts/convert-ly.py
File scripts/convert-ly.py (right):

https://codereview.appspot.com/31830043/diff/1/scripts/convert-ly.py#newcode306
scripts/convert-ly.py:306: last = next_stable
The old code was there so that the \version ".." is updated for a new
stable version <http://codereview.appspot.com/2642041/#msg3>

It looks like the new code does not perform this function.
It takes some patience to understand what this does, either by reading
the doc-string or the code, because it is a sophisticated behavior, and
I do not understand its purpose or motivation.

https://codereview.appspot.com/31830043/