Do a better job at guessing unknown character sets

At least in the kernel development community, we're generally slowly
converting to UTF-8 everywhere, and the old default of Latin1 in emails is
being supplanted by UTF-8, and it doesn't necessarily show up as such in
the mail headers (because, quite frankly, when people send patches
around, they want the email client to do as little as humanly possible
about the patch)

Despite that, it's often the case that email addresses etc still have
Latin1, so I've seen emails where this is a mixed bag, with Signed-off
parts being copied from email (and containing Latin1 characters), and the
rest of the email being a patch in UTF-8.

So this suggests a very natural change: if the target character set is
utf-8 (the default), and if the source already looks like utf-8, just
assume that it doesn't need any conversion at all.

Only assume that it needs conversion if it isn't already valid utf-8, in
which case we (for historical reasons) will assume it's Latin1.

Basically no really _valid_ latin1 will ever look like utf-8, so while
this changes our historical behaviour, it doesn't do so in practice, and
makes the default behaviour saner for the case where the input was already
in proper format.

We could do a more fancy guess, of course, but this correctly handled a
series of patches I just got from Andrew that had a mixture of Latin1 and
UTF-8 (in different emails, but without any character set indication).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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