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

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.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

diffoscope/comparators/xml.py

@@ -85,7 +85,7 @@ class XMLFile(File):
     """
 
     DESCRIPTION = "XML files"
-    FILE_TYPE_RE = re.compile(r"^XML \S+ document")
+    FILE_TYPE_RE = re.compile(r"^(XML \S+ document|GnuCash file)")
 
     @classmethod
     def recognizes(cls, file):