t1300: `--unset-all` can leave an empty section behind (bug)

t1300: `--unset-all` can leave an empty section behind (bug)

We already have a test demonstrating that removing the last entry from a
config section fails to remove the section header of the now-empty
section.

The same can happen, of course, if we remove the last entries in one fell
swoop. This is *also* a bug, and should be fixed at the same time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>