GitDownloadStrategy: fix slow repo_valid? by using -C flag

[ismail]

Summary

repo_valid? runs git status -s without specifying -C, so git falls back to the current working directory. If CWD happens to be a large directory (e.g. the user's home directory), this causes git to scan the entire tree, making the call unexpectedly slow.

Fix by using git -C cached_location status -s so git always operates in the correct directory regardless of CWD.

Test plan

• Verify brew style --fix Library/Homebrew/download_strategy.rb passes
• Verify brew typecheck passes

Before

Takes more than 10 minutes.

; timeout 10m git --git-dir ~/.cache/Homebrew/helix--git/.git status -s
; echo $?
124

After

; time git -C ~/.cache/Homebrew/helix--git status -s
git -C ~/.cache/Homebrew/helix--git status -  0.00s user 0.01s system 121% cpu 0.007 total

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• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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[MikeMcQuaid]

@ismail Can you fill out the pull request template please? Can you provide before and after benchmark examples?

[ismail]

@ismail Can you fill out the pull request template please? Can you provide before and after benchmark examples?

Thanks, updated.