Remove extraneous newlines in pre-receive hook

Found while investigating slow performance in gitlab-org/gitlab-ce#17225

We were adding a newline for every branch push because:

1. GitlabAccess calls changes.lines, which keeps the newline (e.g. ['00000000 12345678 refs/heads/master\n'])
2. GitlabNet calls changes.join("\n"), which adds another newline

lib/gitlab_access.rb

@@ -16,7 +16,7 @@ class GitlabAccess
     @repo_path = repo_path.strip
     @actor = actor
     @repo_name = extract_repo_name(@repo_path.dup, config.repos_path.to_s)
-    @changes = changes.lines
+    @changes = changes.split("\n")
   end
   def exec