Diffs hex dump of ASCII-armoured OpenPGP upstream signature files instead of plain text

This bug was originally reported by Paul Wise (pabs@debian.org) in Debian bug #908991:

diffoscope currently diffs a hex dump of ASCII-armoured OpenPGP
upstream signature files instead of comparing the plain text.

The ASCII-armoured OpenPGP signature files are plain text containing
base64-encoded signatures so it makes more sense to diff the plain text
than a hex dump of the plain text.

For example:

[..]

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