Fix false positive detection when inside submodules

[menehune23]

Summary

Fails detection when .git is a file and not a folder.

Use Cases

Closes #275

Checklist

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• I have added an integration test, if necessary.
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[arjun024]

Hey Andrew 👋
Thanks for the PR. I'm wondering if the right thing to do is for the buildpack to still detect? (but accommodate submodules as well - like git rev-parse HEAD:path-to-sub-module instead of git rev-parse HEAD for example).

@paketo-buildpacks/utilities-maintainers I feel like the use case of extracting git metadata is valid for submodules just like root level git repos. WDYT?

[arjun024]

@menehune23 And would a project.toml exclude give your issue some temporary relief until we figure out the right way to do it

[menehune23]

@menehune23 And would a project.toml exclude give your issue some temporary relief until we figure out the right way to do it

Hey @arjun024, thanks for getting back about this! I do think that project.toml should help alleviate this for now, since the exclude field is respected by kpack (which is ultimately what is running our build).

Feel free to close this if y'all determine this isn't the right approach. Glad it's on the radar now :)