Frescobaldi and git: Flatpak considered harmful

So, I had installed Frescobaldi 4.0.7 on my Linux laptop, using the
flatpak offered by my distro's package manager, and I was about to
learn something new (to me) about flatpak...

Here's a brief report, in the hope that it saves someone else some
trouble:

I keep my Lilypond code under git revision control, and I use the
snippet at
https://wiki.lilypond.community/wiki/Showing_git_commit_hash_(or_other_revision_number)
to make the hash visible in lilypond's output PDFs.  That way, it is
always possible to recover the precise lilypond code that created that
PDF.  But this doesn't work when running Lilypond from within a flatpak
of Frescobaldi.

Although I have not tested any of the other helper applications that
can be configured under Frescobaldi's preferences, it is likely that
the same problem affects many of them.

To cut a long story short, it turns out that Flatpak is designed to
isolate each application within its own "sandbox".  From within a
Flatpak, Frescobaldi is unable to run git to get the hash.  The amount
of sleuthing needed to get from there to here is left to the reader's
imagination.  A word of warning against Flatpak at
https://github.com/frescobaldi/frescobaldi/releases might save someone
a lot of head-scratching in future.

Installing Frescobaldi 4 natively, using the compressed tarball at
github, wasn't that hard, and everything now appears to work OK.

HTH
-- Graham