UnicodeDecodeError on branch names in git loader

https://sentry.softwareheritage.org/organizations/swh/issues/105494/?referrer=phabricator_plugin

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc3 in position 28: invalid continuation byte
  File "swh/loader/core/loader.py", line 414, in load
    self.store_data()
  File "swh/loader/git/base.py", line 105, in store_data
    snapshot = self.get_snapshot()
  File "swh/loader/git/loader.py", line 526, in get_snapshot
    ref_name.decode(),

Loading failure, updating to `failed` status

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Migrated from T4724 (view on Phabricator)