Merged r13831 from trunk to 2.6-stable.

Merged r13831 from trunk to 2.6-stable.

fix Ruby 2.2 warning at lib/redmine/scm/adapters/cvs_adapter.rb.

On ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-linux]:

<pre>
lib/redmine/scm/adapters/cvs_adapter.rb:342:
  warning: Comparable#== will no more rescue exceptions of #<=> in the next release.
lib/redmine/scm/adapters/cvs_adapter.rb:342:
  warning: Return nil in #<=> if the comparison is inappropriate or avoid such comparison.
</pre>

git-svn-id: http://svn.redmine.org/redmine/branches/2.6-stable@13834 e93f8b46-1217-0410-a6f0-8f06a7374b81