`check-shebang-scripts-are-executable` Assumes Git Repo

check-executables-have-shebangs supports being run on files outside of a Git repository, so it seems natural for check-shebang-scripts-are-executable to do the same. I would guess it's also slightly faster to check executable bits through the file system when it can be trusted to track them properly.

Locate the implementations and tests for check-shebang-scripts-are-executable and compare them with check-executables-have-shebangs, beginning with how each hook handles paths outside a Git repository. Done means the executable-checking hook works outside Git and its behavior is covered by tests for filesystem executable bits.