_verifyRefPresent was also broken for tag-based checkouts. Since this
already uses determinePreferredCheckoutSource, which already used
refs/tags/ for tags, this code also doubled-up on tag names.
This whole code path would be a great candidate for that test suite
under t/ if I can ever get it polished up one day :(
To avoid commit spam, I've tested this on my own Qt module directories
by deleting them and forcing them to be checked-out with tags.
Michael Pyne
c9a364c33b