On 1.7.x-issue4460 branch: Partially merge r1561237 from trunk.

Only merged the is_absolute_url() and make_diff_header() parts and not the
new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.7.x-issue4460@1563073 13f79535-47bb-0310-9956-ffa450edef68