handle github assets in git loader

... when we will have a git loader :)

(task by Roberto, moved from trello)

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Migrated from T17 (view on Phabricator)

mentioned in issue swh-storage#3 (closed)

added Git loader priority:Low labels

marked this issue as related to swh/meta#1102

mentioned in issue #2059

mentioned in issue swh/meta#3542 (closed)