git_bare: Send progress updates while cooking
It can be confusing to just have 'Processing...' in the UI for hours. With this change, a new progress update should be sent every few minutes.
In the swh-web UI, it will show only 'Processing... %d objects processed' in the main table, but 'Over %d remaining' will be shown when hovering the progress bar (using a future diff to swh-web). This does not show 'Processing... %d/%d' or a percentage, because the number of remaining objects is not always known.
Resolves T3727
Migrated from D6642 (view on Phabricator)