push pre-commit hook attempts to lint untracked files

[LeoMcA]

Often when switching between local branches I'll have untracked files in my git tree because the .gitignore ignoring these new directories is left in the other branch.

When pushing the pre-commit hook then attempts to lint all these files, which would be fine if it failed fast, but usually it fills my console with spam and locks up while it attempts to lint minified files.

We should ignore untracked files in the push pre-commit hook.

[caugner]

Upstream issue: evilmartians/lefthook#833