Improve computation of numFiles and numDistinctFiles

Optimize the computation of numFiles and numDistinctFiles:
- Compute the list of affected files only once
- Reply on the uniqueness of elements in a set to achieve distinction
rather than folding over a union

Change-Id: Ic9eb4b06a47d6169cd1584406b5bbb19243f670a
diff --git a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/CommitsStatistics.scala b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/CommitsStatistics.scala
index ee230a3..ae1509c 100644
--- a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/CommitsStatistics.scala
+++ b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/CommitsStatistics.scala
@@ -49,20 +49,20 @@
   require(commits.forall(_.botLike == isForBotLike), s"Creating a stats object with isForBotLike = $isForBotLike but containing commits of different type")
   require(commits.forall(_.merge == isForMergeCommits), s"Creating a stats object with isMergeCommit = $isForMergeCommits but containing commits of different type")
+  private lazy val allFiles: List[String] = commits.flatMap(_.files)
     * sum of the number of files in each of the included commits
-  val numFiles: Int =
+  lazy val numFiles: Int = allFiles.size
     * number of distinct files the included commits have been touching
-  val numDistinctFiles: Int = changedFiles.size
+  lazy val numDistinctFiles: Int = allFiles.toSet.size
   def isEmpty: Boolean = commits.isEmpty
-  def changedFiles: Set[String] = union _)
   // Is not a proper monoid since we cannot sum a MergeCommit with a non merge one but it would overkill to define two classes
   def + (that: CommitsStatistics) = {
     require(this.isForMergeCommits == that.isForMergeCommits, "Cannot sum a merge commit stats with a non merge commit stats")