remote.c: hoist read_config into remote_get_1

remote.c: hoist read_config into remote_get_1

Before the previous commit, we had to make sure that
read_config() was called before entering remote_get_1,
because we needed to pass pushremote_name by value. But now
that we pass a function, we can let remote_get_1 handle
loading the config itself, turning our wrappers into true
one-liners.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>