Disable commitWithin when running Reindex

Disable the 'commitWithin' from within Reindex by overriding
the configuration with '-1'. Treat negative values as the
original behavior, auto-flushing but not auto-committing, which
is the least safe but the most efficient for reindexing the
entire site. 

Change-Id: Ifdba797bee871d2a3d8928810a6304bacb850c8c

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

@@ -207,6 +207,10 @@ public class ConfigUtil {
       return defaultValue;
     }
 
+    if (s.startsWith("-")/* negative */) {
+      throw notTimeUnit(section, subsection, setting, valueString);
+    }
+
     try {
       return getTimeUnit(s, defaultValue, wantUnit);
     } catch (IllegalArgumentException notTime) {