config: add test cases for empty value and no value config variables.

config: add test cases for empty value and no value config variables.

The tests in 't1300-repo-config.sh' did not check what happens when
an empty value like the following is used in the config file:

[emptyvalue]
	variable =

Also it was not checked that a variable with no value like the
following:

[novalue]
	variable

gives a boolean "true" value, while an ampty value gives a boolean
"false" value.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>