builtin/send-pack.c: respect user.signingkey
When git-send-pack is exec'ed, as is done by git-remote-http, it
does not read the config, and configured value of user.signingkey is
ignored. Thus it was impossible to specify a signing key over HTTP,
other than the default key in the keyring having a User ID matching
the "Name <email>" format.
This patch at least partially fixes the problem by reading in the GPG
config from within send-pack. It does not address the related problem
of plumbing a value for this configuration option using
`git -c user.signingkey push ...`.
Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>