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>
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>