send-email: fix a "first config key wins" regression in v2.33.0

send-email: fix a "first config key wins" regression in v2.33.0

Fix a regression in my c95e3a3f0b8 (send-email: move trivial config
handling to Perl, 2021-05-28) where we'd pick the first config key out
of multiple defined ones, instead of using the normal "last key wins"
semantics of "git config --get".

This broke e.g. cases where a .git/config would have a different
sendemail.smtpServer than ~/.gitconfig. We'd pick the ~/.gitconfig
over .git/config, instead of preferring the repository-local
version. The same would go for /etc/gitconfig etc.

The full list of impacted config keys (the %config_settings values
which are references to scalars, not arrays) is:

    sendemail.smtpencryption
    sendemail.smtpserver
    sendemail.smtpserverport
    sendemail.smtpuser
    sendemail.smtppass
    sendemail.smtpdomain
    sendemail.smtpauth
    sendemail.smtpbatchsize
    sendemail.smtprelogindelay
    sendemail.tocmd
    sendemail.cccmd
    sendemail.aliasfiletype
    sendemail.envelopesender
    sendemail.confirm
    sendemail.from
    sendemail.assume8bitencoding
    sendemail.composeencoding
    sendemail.transferencoding
    sendemail.sendmailcmd

I.e. having any of these set in say ~/.gitconfig and in-repo
.git/config regressed in v2.33.0 to prefer the --global one over the
--local.

To test this add a test of config priority to one of these config
variables, most don't have tests at all, but there was an existing one
for sendemail.8bitEncoding.

The "git config" (instead of "test_config") is somewhat of an
anti-pattern, but follows established conventions in
t9001-send-email.sh, likewise with any other pattern or idiom in this
test.

The populating of home/.gitconfig and setting of HOME= is copied from
a test in t0017-env-helper.sh added in 1ff750b128e (tests: make
GIT_TEST_GETTEXT_POISON a boolean, 2019-06-21). This test fails
without this bugfix, but now it works.

Reported-by: Eli Schwartz <eschwartz@archlinux.org>
Tested-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>