submodule-config: add subomdule config scope

submodule-config: add subomdule config scope

Before the changes to teach git_config_source to remember scope
information submodule-config.c never needed to consider the question of
config scope.  Even though zeroing out git_config_source is still
correct and preserved the previous behavior of setting the scope to
CONFIG_SCOPE_UNKNOWN, it's better to be explicit about such situations
by explicitly setting the scope.  As none of the current config_scope
enumerations make sense we create CONFIG_SCOPE_SUBMODULE to describe the
situation.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>