tests: simplify "missing PREREQ" message

When a test has no prerequisites satisfied (the usual case), instead
of "missing THING of THING", just say "missing THING".  This does not
affect the output when a test is skipped due to a missing
prerequisites if another prerequisite is satisfied.

For example: instead of

 ok 8 # skip notes work (missing EXPENSIVE of EXPENSIVE)
 ok 9 # skip notes timing with /usr/bin/time (missing EXPENSIVE of USR_BIN_TIME,EXPENSIVE)

write

 ok 8 # skip notes work (missing EXPENSIVE)
 ok 9 # skip notes timing with /usr/bin/time (missing EXPENSIVE of USR_BIN_TIME,EXPENSIVE)

Cc: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -417,8 +417,14 @@ test_skip () {
 	fi
 	case "$to_skip" in
 	t)
+		of_prereq=
+		if test "$missing_prereq" != "$prereq"
+		then
+			of_prereq=" of $prereq"
+		fi
+
 		say_color skip >&3 "skipping test: $@"
-		say_color skip "ok $test_count # skip $1 (missing $missing_prereq of $prereq)"
+		say_color skip "ok $test_count # skip $1 (missing $missing_prereq${of_prereq})"
 		: true
 		;;
 	*)