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>

builtin/send-pack.c

@@ -11,6 +11,7 @@
 #include "transport.h"
 #include "version.h"
 #include "sha1-array.h"
+#include "gpg-interface.h"
 
 static const char send_pack_usage[] =
 "git send-pack [--all | --mirror] [--dry-run] [--force] [--receive-pack=<git-receive-pack>] [--verbose] [--thin] [--atomic] [<host>:]<directory> [<ref>...]\n"
@@ -113,6 +114,8 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	int from_stdin = 0;
 	struct push_cas_option cas = {0};
 
+	git_config(git_gpg_config, NULL);
+
 	argv++;
 	for (i = 1; i < argc; i++, argv++) {
 		const char *arg = *argv;