Refactor XDocServlet: Move revision resolution into helper method

Change-Id: I88bbc7d9a6fc3793ad963261d5e64057969c9cb7
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

src/main/java/com/googlesource/gerrit/plugins/xdocs/XDocServlet.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/xdocs/XDocServlet.java b/src/main/java/com/googlesource/gerrit/plugins/xdocs/XDocServlet.java
index e6fa865..8bb64c5 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/xdocs/XDocServlet.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/xdocs/XDocServlet.java
@@ -130,12 +130,7 @@
 
       Repository repo = repoManager.openRepository(key.project);
       try {
-        ObjectId revId =
-            repo.resolve(MoreObjects.firstNonNull(rev, Constants.HEAD));
-        if (revId == null) {
-          Resource.NOT_FOUND.send(req, res);
-          return;
-        }
+        ObjectId revId = resolveRevision(repo, rev);
 
         if (ObjectId.isId(rev)) {
           RevWalk rw = new RevWalk(repo);
@@ -280,6 +275,16 @@
     return rev;
   }
 
+  private static ObjectId resolveRevision(Repository repo, String revision)
+      throws ResourceNotFoundException, IOException {
+    ObjectId revId =
+        repo.resolve(MoreObjects.firstNonNull(revision, Constants.HEAD));
+    if (revId == null) {
+      throw new ResourceNotFoundException();
+    }
+    return revId;
+  }
+
   private static String computeETag(Project.NameKey project, ObjectId revId,
       String file) {
     return Hashing.md5().newHasher()