fetch tests: test --prune and refspec interaction

fetch tests: test --prune and refspec interaction

Add a test for the interaction between explicitly provided refspecs
and fetch.prune.

There's no point in adding this boilerplate to every combination of
unset/false/true, it's instructive and sufficient to show that no
matter if the variable is unset, false or true the refspec on the
command-line overrides any configuration variable.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>