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 swh-web#2782 (closed)
Depends on !625 (closed)
Migrated from D5208 (view on Phabricator)