Add REST endpoint to get content of a file in a commit

It is already possible to get the content of a file from the HEAD
revision of a branch. Add the same functionality for arbitrary
commits.

Change-Id: Id8e48bb40b4f383997aa9b83440a57ab90e763ee
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/project/Module.java

@@ -67,6 +67,7 @@ public class Module extends RestApiModule {
 
     child(PROJECT_KIND, "commits").to(CommitsCollection.class);
     get(COMMIT_KIND).to(GetCommit.class);
+    child(COMMIT_KIND, "files").to(FilesInCommitCollection.class);
 
     child(PROJECT_KIND, "dashboards").to(DashboardsCollection.class);
     get(DASHBOARD_KIND).to(GetDashboard.class);