lib/batcher: log when committing uploads has finished - fixes #5564 - #10057

What does this change do?

When the batcher shuts down it logs Committing uploads - please wait... and then nothing more, so the log looks as if rclone stopped part way through. As suggested in the issue, this adds a new message after b.wg.Wait(), once the outstanding batches have been committed. The existing message is unchanged:

INFO  : Dropbox root 'x': Committing uploads - please wait...
INFO  : Dropbox root 'x': Committing uploads - done

• The new message is logged at INFO, like the existing one.
• The batcher now lives in lib/batcher, so the change covers both dropbox and googlephotos.
• "done" means the commit has finished. If a batch failed, that error is still logged above this line, as before.

The bisync test log filter drops the existing message, so I added the new one next to it. Without that, the bisync golden tests would fail on dropbox.

Testing

• TestBatcherShutdownLog commits an item to an async batcher and shuts it down. It checks that the new message is logged after the batch has been committed. It fails on master.
• These pass: go build ./..., make quicktest, go test ./lib/batcher/ ./cmd/bisync/, go test -race ./lib/batcher/, go vet and golangci-lint run ./lib/batcher/... ./cmd/bisync/....
• I don't have dropbox or Google Photos accounts, so I haven't run test_all against them. The change is in the shared batcher and is covered by the test above.

Linked issue

Fixes #5564

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate. (No doc change needed: log output only.)
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend. n/a, not a backend change.
• This Pull Request is ready for review.