comparators: xml: also recognize "GnuCash file" as XML

Mattia Rizzolorequested to merge
mattia/diffoscope:gnucash into master

file says:

% file foo.gnucash
foo.gnucash: GnuCash file

gnucash files are, by default, gz-compressed, and file recognizes them as such:

% file main.gnucash
main.gnucash: gzip compressed data, from Unix, original size modulo 2^32 1330067

But once uncompressed they are normal XML files, but somehow file has its own filetype for them.

I'm pushing it as a MR as I'd like to check with others that this is fine here :)

Merge request reports