Exclude files that don't exist from applicable_files

[jawshooah]

Fixes #219

[lencioni]

👍 want to add a line to the changelog while you are at it?

[jawshooah]

Done!

[lencioni]

Thanks for the code @jawshooah!

[sds]

Thanks for the pull request, @jawshooah.

On deeper thought, I think I merged this too early. This doesn't solve the underlying problem, which is that HookContext::PreCommit#modified_files returns non-existent files. We should probably hoist a fix close to the source of the problem to properly fix this. I'm happy to look into that later.

[sds]

Closing the loop on this to say I implemented a more thorough fix with a test in b450815.