git_objects: Use raw offset_bytes to format dates, and remove format_offset()
This is needed when recomputing the manifest of git objects with weird offsets.
Part of T3752.
Migrated from D6890 (view on Phabricator)
This is needed when recomputing the manifest of git objects with weird offsets.
Part of T3752.
Migrated from D6890 (view on Phabricator)