1. short hashes now work
2. print the long hash in addition to the detected cabal version of HLS
3. add `--git-describe-version` switch as an alternative to
   `--overwrite-version`

Fix 1. and 2. for GHC as well.

Typo

A one word tweak to weaken the language in the initial explanation
to make it "less scary": in general ghcup does not always download
all of ghcup, ghc, cabal, stack, and hls
(unless requested or they are not already installed, etc),
but "will download" sounds like the user is has no choice here
except to always download everything,
which might give them second thoughts about trying this script
and hence adopting ghcup.

Perhaps the wording could be made further more precise,
but at least "can" gives one less anxiety.