assistant: Always batch changes found in startup scan.

Batch detection is heuristic, so can sometimes fail. I observed one such
failure while starting up in a repository with 87000 files. After the first
several batches of ~5000 files, it fell out of batch mode, and never
re-entered it, and so made many more commits of a few files at a time
than necessary.

So, let's always use batch mode when in the startup scan. This avoids the
heuristic there, at least.

There is clearly also room to improve the heuristic. Possibly 10 files is
too high a bar to be found during a commit, on a system that can commit
quickly.

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