pre-push hook should only look at staged changes

[jyn514]

if i have modified the repository locally, but not committed those changes, and i run git push, i expect it to lint on the changes that are actually going to be pushed, not the ones i have laying around.

instead, src/etc/pre-push lints on the working directory.

it would be nice if it checked if there are unstaged changes, and if so copied the working directory to a tempdir to run the checks there. here is a hacky bash script that does it for checks that work file-at-a-time; i don't think tidy ever looks at multiple files at once or tries to build code, but if it does it would need more work.

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