Ignore GerritServer.properties at the top level

This might exist if the developer is trying to test directly from
the source directory by running something like:

  mvn package && java -jar target/gerrit-*.war foo

and the "foo" subcommand requires GerritServer.properties to speak
to the database.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/.gitignore b/.gitignore
index 834721e..43d2b7b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /target
+/GerritServer.properties
 /.classpath
 /.project
 /.settings/org.maven.ide.eclipse.prefs