[config, merge] Validate username usage in merge source URL vs working copy URL

[aik099]

When --merge-source-url option of merge command (or connected config property for that working copy) has username specified (e.g. svn://user@domain.com/svn/repository/trunk), but working copy URL doesn't contain username (e.g. svn://user@domain.com/svn/repository/tags/stable), then svn merge command considers this as an external merge and won't populate svn:mergeinfo property during merge.

TODO:

1. determine what we use as working copy identifier for config setting storage (just domain + path or also protocol or user)
2. validate, that source/target merge URLs start with the same thing (same protocol, same user/password, same domain)
   • in the «\ConsoleHelpers\SVNBuddy\Command\MergeCommand::performMerge» method (or earlier)
   • when changing «merge.source-url» config setting