Speed up migration squasher

What does this MR do and why?

This speeds up the migration squasher (rake gitlab:db:squash) by skipping to pass files that don’t exist to 'git rm'.

On my laptop, the task takes about 265s before the change, which drops down to about 38.033s ± 0.658s after the change. I didn’t measure a full hyperfine run because it would've taken over half an hour. About a 86% drop should be significant enough though.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run bundle exec rake gitlab:db:squash\[origin/16-3-stable-ee\]
2. See that it finishes in less than half a minute