checkpatch: detect doubly-encoded UTF-8

checkpatch: detect doubly-encoded UTF-8

Copy and pasting from Thunderbird's "view source" window results in double
encoding of multibyte UTF-8 sequences.  The appearance of those sequences is
very peculiar, so detect it and give an error despite the (low) possibility
of false positives.

As the major offender, I am also adding the same check to my applypatch-msg
and commit-msg hooks, but this will also cause patchew to croak loudly when
this mistake happens.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1558099140-53240-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>