Improve detection of modified packages
Summary:
Ignore root files like composer.json: awk splits fields with path separator
and only keep lines with more than one field, so at least a directory.
Consolidate all operations in an awk script.
Consolidate sort and uniq using sort -u.
Test Plan: ./_utils/repo/get-repos-modified-by-last-commit.sh <several hashes>
Differential Revision: https://devcentral.nasqueron.org/D3786