Only consider tracked files when building toolchain hashes

[ahal]

Previously we were walking the file system and were e.g scanning files in the .git directory. This can be quite slow on larger repos.

Fixes #663

[jcristau]

@glandium fyi

[ahal]

Note that gecko_taskgraph already does this, but it uses FileFinder so I couldn't port the fix exactly.

[gregtatum]

Thanks for the work on this, this will be a big quality of life improvement for translations!