GHC package mirror copies extra branches and doesn't remove them.

For example

• https://github.com/haskell/unix/branches has 6 branches, yet
• https://gitlab.haskell.org/ghc/packages/unix/-/branches has 23 (all the dependabot) branches.

I think short term solution would be to not clone dependabot/* branches.

haddock and bytestring seem to use dependabot as well. cc @bgamari @Bodigrim

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For the record, I asked about cleanup of wip branches long ago. But let have a timestamp for that too.