This info is indeed valuable on top of the commit hash, thanks to @jfbu
who complained about its absence.

With commit 8c0dd0ebbedd kern accesses were fixed but the results never
tested for the lookup, which caused the default which is set up before
to be nil’ed. Fix this by reusing the value we initialized the kern
factor to.

With the new loader, the ``units_per_em`` field resides under the
toplevel TFM structure with the key ``units``. For AFM fonts, it is not
fount under the metadata table too, which we are currently querying.
Thus we prefer the main value, falling back on metadata only in case it
is missing.

At this occasion, tidy up our unit lookup helper and use that wherever
we need access to the values ourselves.

Defining a combination did not override the default file: lookup so
every font defined that way will at font embedding time trigger a reload
of the index on account of a missing file. Setting the ``combo:`` lookup
will prevent the issue.

Due to the origin in other parts of Luatex, some messages in this file
were logged under the “load” facility; corrected. Also, the combo
mechanism still spits debug levels of noise at log level zero; raised to
two.

Adress issue #350

For some reason our stack copy of the current node gets double-freed.
Since the “components” field will probably vanish soon, we can get by
for now by skipping the explicit free.

This could become the common check eventually.

The version check was removed by accident in commit 5ce60cc98a30. This
isn’t normally a problem except when people start to run the Pretest
version on ancient Luatex binaries … In any case, the check must be
present. While we’re at it, make the error messages consistent between
the tool and a live TeX run. This can’t cover the fact that there is no
(direct) means of determining the Luatex version when not running in TeX
mode, so the checks still aren’t equivalent.

Fix issue #351

The letterspace node handler dereferences kern values of non-existant
character information under some circumstances, causing Luatex to crash.
To avoid this crash, check more thoroughly for the presence of the
fields accessed.

Discovered by @schlcht