Gracefully handle commits where files are missing from git

[relud]

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

probe-scraper/probe_scraper/scrapers/git_scraper.py

Line 128 in c71f5dd

contents = get_file_at_hash(repo, _hash, rel_path)

probe-scraper/probe_scraper/scrapers/git_scraper.py

Lines 84 to 85 in c71f5dd

	def get_file_at_hash(repo, _hash, filename):
	return repo.git.show("{hash}:{path}".format(hash=_hash, path=filename))

the current workaround requires listing every commit where the file was deleted:

probe-scraper/probe_scraper/scrapers/git_scraper.py

Lines 35 to 49 in c71f5dd

	# Some commits in projects might contain invalid metric files.
	# When we know these problems are fixed in later commits we can skip them.
	SKIP_COMMITS = {
	"engine-gecko": [
	"9bd9d7fa6c679f35d8cbeb157ff839c63b21a2e6" # Missing schema update from v1 to v2
	],
	"engine-gecko-beta": [
	"9bd9d7fa6c679f35d8cbeb157ff839c63b21a2e6" # Missing schema update from v1 to v2
	],
	"gecko": [
	"43d8cf138695faae2fca0adf44c94f47fdadfca8", # Missing gfx/metrics.yaml
	"340c8521a54ad4d4a32dd16333676a6ff85aaec2", # Missing toolkit/components/glean/pings.yaml
	"4520632fe0664572c5f70688595b7721d167e2d0", # Missing toolkit/components/glean/pings.yaml
	],
	}

probe-scraper/probe_scraper/scrapers/git_scraper.py

Line 103 in c71f5dd

skip_commits = SKIP_COMMITS.get(repo_info.name, [])

probe-scraper/probe_scraper/scrapers/git_scraper.py

Lines 123 to 124 in c71f5dd

	if _hash in skip_commits:
	continue

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.