Add SSH command to run Git garbage collection

Add a new SSH command that allows to run the Git garbage collection
for specific or all Gerrit projects.

The default parameters for the Git garbage collection can be defined
in the user global Git configuration file of the system user that
runs the Gerrit server.

It is possible to specify repository specific parameters for the
garbage collection in the Git configuration files on repository
level.

All GC runs are logged in a newly added GC log file. This log file
also contains statistics of the repositories that were garbage
collected.

Change-Id: Ie8730e3db785d5e05d5b739cfb1ef87ba515e870
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollectionQueue.java

@@ -0,0 +1,42 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.git;
+
+import com.google.common.collect.Sets;
+import com.google.gerrit.reviewdb.client.Project;
+import com.google.inject.Singleton;
+
+import java.util.Collection;
+import java.util.Set;
+
+@Singleton
+class GarbageCollectionQueue {
+  private final Set<Project.NameKey> projectsScheduledForGc = Sets.newHashSet();
+
+  synchronized Set<Project.NameKey> addAll(Collection<Project.NameKey> projects) {
+    Set<Project.NameKey> added =
+        Sets.newLinkedHashSetWithExpectedSize(projects.size());
+    for (Project.NameKey p : projects) {
+      if (projectsScheduledForGc.add(p)) {
+        added.add(p);
+      }
+    }
+    return added;
+  }
+
+  synchronized void gcFinished(Project.NameKey project) {
+    projectsScheduledForGc.remove(project);
+  }
+}