avoid making empty commits

This doesn't avoid it sometimes attempting to commit when there are no
changes. Typically that happens when a change is pushed in from another
repo; the watcher sees the file and tries to stage it, resulting in an
empty commit. Really fixing that would probably use more CPU than
occasionally trying to make an empty commit.

However, this does save a lot of unnecessary work, as those empty commits
had to be synced out, which no longer happens.

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