Gracefully handle commits where files are missing from git

[relud]

this code fails when there are commits where a file was deleted:

https://github.com/mozilla/probe-scraper/blob/c71f5dd8b0f4a98a9c2de83b38f22ff4e0b40205/probe_scraper/scrapers/git_scraper.py#L128 https://github.com/mozilla/probe-scraper/blob/c71f5dd8b0f4a98a9c2de83b38f22ff4e0b40205/probe_scraper/scrapers/git_scraper.py#L84-L85

the current workaround requires listing every commit where the file was deleted: https://github.com/mozilla/probe-scraper/blob/c71f5dd8b0f4a98a9c2de83b38f22ff4e0b40205/probe_scraper/scrapers/git_scraper.py#L35-L49 https://github.com/mozilla/probe-scraper/blob/c71f5dd8b0f4a98a9c2de83b38f22ff4e0b40205/probe_scraper/scrapers/git_scraper.py#L103 https://github.com/mozilla/probe-scraper/blob/c71f5dd8b0f4a98a9c2de83b38f22ff4e0b40205/probe_scraper/scrapers/git_scraper.py#L123-L124

That workaround is still needed for invalid files, but for missing files we should add code to handle the exception from git, probably with continue.