git-config: do not forget seeing "a.b.var" means we are out of "a.var" section.

git-config: do not forget seeing "a.b.var" means we are out of "a.var" section.

Earlier code tried to be half-careful and knew the logic that
seeing "a.var" after seeing "a.b.var" is a sign of the previous
"a.b." section has ended, but forgot it has to handle the other
way.  Seeing "a.b.var" after seeing "a.var" is a sign that "a."
section has ended, so a new "a.var2" variable should be added
before the location "a.b.var" appears.

Signed-off-by: Junio C Hamano <junkio@cox.net>