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storage: Allow to filter out branches by prefix when counting them

Description

storage: Allow to filter out branches by prefix when counting them

Add an optional branch_name_exclude_prefix parameter to the
snapshot_count_branches method of the Storage interface.

It enables to filter out branches whose name starts with a
given prefix when counting.

The purpose is to get accurate counters in swh-web as pull
request branches will be filtered out by default.

Related to T2782