download_strategy: silence detached head warning on git clone

[dtrodrigues]

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• Have you successfully run brew tests with your changes locally?

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Currently, when building a formula from a git repository, a git detached head warning is displayed:

$ rm -rf $(brew --cache)/dive--git && brew install -s dive
==> Cloning https://github.com/wagoodman/dive.git
Cloning into '/Users/username/Library/Caches/Homebrew/dive--git'...
Note: switching to '0872cc18d44a96ed9f59202ac95c556f7e7919a7'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

==> Checking out tag v0.9.2
HEAD is now at 0872cc1 cut release
==> go build -ldflags -s -w -X main.version=0.9.2
...

After the change:

 $ rm -rf $(brew --cache)/dive--git && brew install -s dive
==> Cloning https://github.com/wagoodman/dive.git
Cloning into '/Users/username/Library/Caches/Homebrew/dive--git'...
==> Checking out tag v0.9.2
HEAD is now at 0872cc1 cut release
==> go build -ldflags -s -w -X main.version=0.9.2
...

[maxim-belkin]

I like the change but I wonder if it should be on a separate line with a comment.

[MikeMcQuaid]

Very nice 👏🏻