send-email: fix regression in sendemail.identity parsing

Fix a regression in my recent 3494dfd3ee ("send-email: do defaults ->
config -> getopt in that order", 2019-05-09). I missed that the
$identity variable needs to be extracted from the command-line before
we do the config reading, as it determines which config variable we
should read first. See [1] for the report.

The sendemail.identity feature was added back in
34cc60ce2b ("send-email: Add support for SSL and SMTP-AUTH",
2007-09-03), there were no tests to assert that it worked properly.

So let's fix both the regression, and add some tests to assert that
this is being parsed properly. While I'm at it I'm adding a
--no-identity option to go with --[to|cc|bcc] variable, since the
semantics are similar. It's like to/cc/bcc except that unlike those we
don't support multiple identities, but we could now easily add it
support for it if anyone cares.

In just fixing the --identity command-line parsing bug I discovered
that a narrow fix to that wouldn't do. In read_config() we had a state
machine that would only set config values if they weren't set already,
and thus by proxy we wouldn't e.g. set "to" based on sendemail.to if
we'd seen sendemail.gmail.to before, with --identity=gmail.

I'd modified some of the relevant code in 3494dfd3ee, but just
reverting to that wouldn't do, since it would bring back the
regression fixed in that commit.

Refactor read_config() do what we actually mean here. We don't want to
set a given sendemail.VAR if a sendemail.$identity.VAR previously set
it. The old code was conflating this desire with the hardcoded
defaults for these variables, and as discussed in 3494dfd3ee that was
never going to work. Instead pass along the state of whether an
identity config set something before, as distinguished from the state
of the default just being false, or the default being a non-bool or
true (e.g. --transferencoding).

I'm still not happy with the test coverage here, e.g. there's nothing
testing sendemail.smtpEncryption, but I only have so much time to fix
this code.

1. https://public-inbox.org/git/5cddeb61.1c69fb81.47ed4.e648@mx.google.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
