Update SvnDownloader.php

added --ignore-ancestry to the switch statement because it can be a problem with svn:properties
Throw RuntimeException when ambiguous reference was found.
improve exception message
fix plural in exception message
use statement
remove duplicity
show warning only instead of throwing exception
Update SvnDownloader.php
Merge remote-tracking branch 'sebbrandt87/master'
Refactor code so it works cross-platform, refs #2825
Attempt using bash on windows if it is available, fixes #2831
Merge remote-tracking branch 'johnikx/ambiguous-reference-warning'
Refactor to avoid global static state
Fix test because the order of filesystem iteration is non deterministic