remote.c: do not trigger remote.<name>.<var> codepath for two-level names

If the config file contains a section like this:

[remote]
default = foo

(it should be '[remotes]') then commands like

git status
git checkout
git branch -v

fail even though they are not obviously related to remotes. (These
commands write "ahead, behind" information and, therefore, access the
per-remote information).

Unknown configuration keys should be ignored, not trigger errors.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>