branch test: fix invalid config key access

branch test: fix invalid config key access

Fixes the test by changing "branch.s/s/dummy" to "branch.s/s.dummy" which is
the right way of accessing config key "branch.s/s.dummy". Purpose of
this test is to confirm that this key doesn't exist after the branch
"s/s" has been renamed to "s".

Earlier it was trying to access invalid config key and hence was getting
an error. However, this wasn't caught because we were expecting the
command to fail for other reason as mentioned above.

Signed-off-by: Sahil Dua <sahildua2305@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>