loader: Ensure to fetch latest snapshot produced by a git visit type

SWH data model allows an origin to have multiple visit types, in particular
a git origin can have visit types 'git' and 'git-checkout'.

We must ensure to retrieve the latest snapshot for a git visit type in the
git loader implementation as it can break incremental loading of a git origin
having both visit types mentioned above.

Indeed a 'git-checkout' visit type produces a snapshot with a single branch
while a 'git' visit type produces a snapshot containing all branches of the
loaded repository. Previously, if the latest snapshot retrieved was produced
by a 'git-checkout' visit type, the loader would refetch all branches and
associated git objects while most of them have already been archived.

Related to swh/meta#5092.