Correctly handle the result of a DeferredList in svn_buildbot.py

Fixes #2766.
backport buildbot.util.identifiers from nine
Unique-ify user identifiers when attributes do not agree.

This solves the problem when two users happen to have the same
"suggested" identifier (the first argument to findUserByAttr).
Fixes #2587.
identifiers aren't defined in this version
Merge branch 'buildbot-0.8.9'