push: drop confusing configset/callback redundancy

push: drop confusing configset/callback redundancy

We parse push config by calling git_config() with our git_push_config()
callback. But inside that callback, when we see "push.gpgsign", we
ignore the value passed into the callback and instead make a new call to
git_config_get_value().

This is unnecessary at best, and slightly wrong at worst (if there are
multiple instances, get_value() only returns one; both methods end up
with last-one-wins, but we'd fail to report errors if earlier
incarnations were bogus).

The call was added by 68c757f219 (push: add a config option push.gpgSign
for default signed pushes, 2015-08-19). That commit doesn't give any
reason to deviate from the usual strategy here; it was probably just
somebody unfamiliar with our config API and its conventions.

It also added identical code to builtin/send-pack.c, which also handles
push.gpgsign.

And then the same issue spread to its neighbor in b33a15b081 (push: add
recurseSubmodules config option, 2015-11-17), presumably via
cargo-culting.

This patch fixes all three to just directly use the value provided to
the callback. While I was adjusting the code to do so, I noticed that
push.gpgsign is overly careful about a NULL value. After
git_parse_maybe_bool() has returned anything besides 1, we know that the
value cannot be NULL (if it were, it would be an implicit "true", and
many callers of maybe_bool rely on that). Here that lets us shorten "if
(v && !strcasecmp(v, ...))" to just "if (!strcasecmp(v, ...))".

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