config: forbid newline as core.commentChar

Since we usually look for a comment char while parsing line-oriented
files, setting core.commentChar to a single newline can confuse our code
quite a bit. For example, using it with "git commit" causes us to fail
to recognize any of the template as comments, including it in the config
message. Which kind of makes sense, since the template content is on its
own line (so no line can "start" with a newline). In other spots I would
not be surprised if you can create more mischief (e.g., violating loop
assumptions) but I didn't dig into it.

Since comment characters are a local preference, to some degree this is
a case of "if it hurts, don't do it". But given that this would be a
silly and pointless thing to do, and that it makes it harder to reason
about code parsing comment lines, let's just forbid it.

There are other cases that are perhaps questionable (e.g., setting the
comment char to a single space), but they seem to behave reasonably (at
least a simple "git commit" will correctly identify and strip the
template lines). So I haven't worried about going on a hunt for every
stupid thing a user might do to themselves, and just focused on the most
confusing case.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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