config.c: Escape backslashes in section names properly

config.c: Escape backslashes in section names properly

If an element of the configuration key name other than the first or last
contains a backslash, it is not escaped on output, but is treated as an
escape sequence on input. Thus, the backslash is lost when re-loading
the configuration.

This patch corrects this by having backslashes escaped properly, and
introduces a new test for this bug.

Signed-off-by: Bryan Donlan <bdonlan@fushizen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>