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

t/t5510-fetch.sh

@@ -609,6 +609,10 @@ test_configured_prune () {
 test_configured_prune unset unset kept   kept   ""
 test_configured_prune unset unset kept   kept   "--no-prune"
 test_configured_prune unset unset pruned kept   "--prune"
+test_configured_prune unset unset kept   pruned \
+	"--prune origin refs/tags/*:refs/tags/*"
+test_configured_prune unset unset pruned pruned \
+	"--prune origin refs/tags/*:refs/tags/* +refs/heads/*:refs/remotes/origin/*"
 
 test_configured_prune false unset kept   kept   ""
 test_configured_prune false unset kept   kept   "--no-prune"
@@ -625,6 +629,10 @@ test_configured_prune unset false pruned kept   "--prune"
 test_configured_prune false false kept   kept   ""
 test_configured_prune false false kept   kept   "--no-prune"
 test_configured_prune false false pruned kept   "--prune"
+test_configured_prune false false kept   pruned \
+	"--prune origin refs/tags/*:refs/tags/*"
+test_configured_prune false false pruned pruned \
+	"--prune origin refs/tags/*:refs/tags/* +refs/heads/*:refs/remotes/origin/*"
 
 test_configured_prune true  false kept   kept   ""
 test_configured_prune true  false pruned kept   "--prune"
@@ -641,6 +649,10 @@ test_configured_prune false true  pruned kept   "--prune"
 test_configured_prune true  true  pruned kept   ""
 test_configured_prune true  true  pruned kept   "--prune"
 test_configured_prune true  true  kept   kept   "--no-prune"
+test_configured_prune true  true  kept   pruned \
+	"--prune origin refs/tags/*:refs/tags/*"
+test_configured_prune true  true  pruned pruned \
+	"--prune origin refs/tags/*:refs/tags/* +refs/heads/*:refs/remotes/origin/*"
 
 test_expect_success 'all boundary commits are excluded' '
 	test_commit base &&