checkpatch: check utf-8 content from a commit log when it's missing from charset

checkpatch: check utf-8 content from a commit log when it's missing from charset

Check that a commit log doesn't contain UTF-8 when a mail header
explicitly defines a different charset, like

'Content-Type: text/plain; charset="us-ascii"'

Signed-off-by: Pasi Savanainen <pasi.savanainen@nixu.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Message-id: 20180430124651.10340-3-stefanha@redhat.com
Cc: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit fa64205df9dfd7b7662cc64a7e82115c00e428e5)
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>