utils/git_repository: use Pathname.pwd if no repo specified

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This will allow formulae to do Utils.git_head instead of Utils.git_head(buildpath)
This is useful for cd blocks such as:

cd "resource-cloned-from-git" do
  resource_commit = Utils.git_head
  # ...
end

[BrewTestBot]

Review period will end on 2021-01-13 at 19:10:17 UTC.

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Review period ended.