bug#19950: 25.0.50; Gnus nnimap -- don't split messages back into original group

[Eric Abrahamsen]

Presently, `nnimap-split-incoming-mail' will split a message back into
the group it was already in, if that's how the split rules come out.
That results in a useless COPY/EXPUNGE, effectively incrementing the
message's UID to no purpose.

It's a small inefficiency, and doesn't happen that often, but if a user
has otherwise misconfigured bits of his/her imap setup (ahem), it can
lead to some strange behavior. It shouldn't have any real downside,
either.

I've attached a patch that does this, and can push it if it's okay. The
only potential problem I can imagine would be if an utf7-encoded group
name were somehow compared against a non-encoded group name, but I don't
see how that could happen.

If this is acceptable, does it need a CHANGELOG mention? I've gotten
pretty confused about what's LOG-worthy, and what isn't.

0001-Don-t-split-imap-messages-back-into-original-group.patch
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