Fix git fetching so that it actually downloads.

My reading of the git-fetch manpage left me with the idea that adding
the '-t' (for --tag) option would additionally cause all tags to be
fetched from the given repository. The manpage doesn't actually say this
though, and git apparently instead downloads just the tags (and needed
objects), so this change has caused master branches around the
kdesrc-build world to stop updating. :(

The idea behind this had been to ensure that if the 'tag' option were
used in kdesrc-build that git would have the most up-to-date list of
tags available, but I guess we'll just have to go back to
crossing-our-fingers and hoping git already knows about the tag for now.