send-email: lazily load config for a big speedup

Reduce the time it takes git-send-email to get to even the most
trivial of tasks (such as serving up its "-h" output) by first listing
config keys that exist, and only then only call e.g. "git config
--bool" on them if they do.

Over a lot of runs this speeds the time to "-h" up for me from ~250ms
to ~150ms, and the runtime of t9001-send-email.sh goes from ~25s to
~20s.

This introduces a race condition where we'll do the "wrong" thing if a
config key were to be inserted between us discovering the list and
calling read_config(), i.e. we won't know about the racily added
key. In theory this is a change in behavior, in practice it doesn't
matter.

The config_regexp() function being changed here was added in
dd84e528a34 (git-send-email: die if sendmail.* config is set,
2020-07-23) for use by git-send-email. So we can change its odd return
value in the case where no values are found by "git config". The
difference in the *.pm code would matter if it was invoked in scalar
context, but now it no longer is.

Arguably this caching belongs in Git.pm itself, but in lieu of
modifying it for all its callers let's only do this for "git
send-email". The other big potential win would be "git svn", but
unlike "git send-email" it doesn't check tens of config variables one
at a time at startup (in my brief testing it doesn't check any).

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

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