test-lib: Print missing prerequisites in test output

Change the test output to print needed prerequisites as part of the
TAP. This makes it easy to see at a glance why a test was
skipped. Before:

    ok 7 # skip <message>
    ok 9 # skip <message>

After:

    ok 7 # skip <message> (prereqs: DONTHAVEIT)
    ok 9 # skip <message> (prereqs: HAVEIT,DONTHAVEIT)

This'll also be useful for smoke testing output, where the developer
reading the output may not be familiar with the system where tests are
being skipped.

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

t/test-lib.sh

@@ -403,7 +403,7 @@ test_skip () {
 	case "$to_skip" in
 	t)
 		say_color skip >&3 "skipping test: $@"
-		say_color skip "ok $test_count # skip $1"
+		say_color skip "ok $test_count # skip $1 (prereqs: $prereq)"
 		: true
 		;;
 	*)