A change in Git tag naming

[Johan Tibell]

Hi,

Since Cabal and cabal-install might not be released at the same time
and thus not from the same Git commit we will from now on tag releases
in the Git repo as

Cabal-vX.Y.Z
and
cabal-install-vX.Y.Z

The "v" in the tag is used to distinguish the tags from branches we
might create some time before a release. Such branches would be named
e.g. Cabal-1.16.

to keep track of what was actually released.

-- Johan