[merge] Display progress bar for getting mergeable revisions

[aik099]

The result shown in * Upcoming Merge Status ... line provided via \ConsoleHelpers\SVNBuddy\Command\MergeCommand::calculateUsableRevisions method can take some time (1 minute for large projects) to calculate and that time is spent on calling svn mergeinfo command on remote SVN server.

Since the command doesn't provide any progress we need to display that progress ourselves as a never-ending progress bar or second counter while the command is running.