Correctly use configPath specified on the command line

Adding -Dcom.google.gitiles.configPath=<path> previously did not
supersede the default from the packaged WEB-INF/web.xml.

Change-Id: I334ee44a1d9748385744f70bd27badb3bcddfd47

gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
index 640f3ca..b7d9a10 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesConfig.java
@@ -34,13 +34,10 @@
   }
 
   public static File defaultFile(FilterConfig filterConfig) {
-    String configPath = null;
-    if (filterConfig != null) {
+    String configPath = System.getProperty(PROPERTY_NAME, DEFAULT_PATH);
+    if (configPath == null && filterConfig != null) {
       configPath = filterConfig.getInitParameter(FILTER_CONFIG_PARAM);
     }
-    if (configPath == null) {
-      configPath = System.getProperty(PROPERTY_NAME, DEFAULT_PATH);
-    }
     return new File(configPath);
   }