Allow time unit variables to be 0

plugins.checkFrequency can be disabled by setting the time to 0,
but this is not allowed in a time unit typed variable due to the
regex requiring the value to start with a digit in the range 1-9.
Permit 0 as a value.

Change-Id: I2deca08fe9b0ea93ef23f626fc44cffe98ad2d95

gerrit-server/src/main/java/com/google/gerrit/server/config/ConfigUtil.java

@@ -228,7 +228,7 @@ public class ConfigUtil {
    */
   public static long getTimeUnit(final String valueString, long defaultValue,
       TimeUnit wantUnit) {
-    Matcher m = Pattern.compile("^([1-9][0-9]*)\\s*(.*)$").matcher(valueString);
+    Matcher m = Pattern.compile("^(0|[1-9][0-9]*)\\s*(.*)$").matcher(valueString);
     if (!m.matches()) {
       return defaultValue;
     }