test-lib: Simplify test counting.

Since the test case counter was incremented very late, there were a few
users of the counter had to do their own incrementing. Now we increment it
early and simplify these users.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

t/t5515-fetch-merge-logic.sh

@@ -129,8 +129,7 @@ do
 	'' | '#'*) continue ;;
 	esac
 	test=`echo "$cmd" | sed -e 's|[/ ][/ ]*|_|g'`
-	cnt=`expr $test_count + 1`
-	pfx=`printf "%04d" $cnt`
+	pfx=`printf "%04d" $test_count`
 	expect_f="$TEST_DIRECTORY/t5515/fetch.$test"
 	actual_f="$pfx-fetch.$test"
 	expect_r="$TEST_DIRECTORY/t5515/refs.$test"