[Bsb] Better dry run feature

[chenglou]

The current bsb -- -n ninja dry run feature works well, but still potentially outputs lots of stuff when there are stale files or errors in the project.

@bobzhang I don't mind the extra output since this is primarily used by the editor to check whether the output is empty. But I'm guessing the intention of having a specialized dry run is better speed?

[bobzhang]

@chenglou yes. Is it a high priority

[chenglou]

Ah, also, bsb -- -n wipes .compiler.log and leaves only # Done, as if the build succeeded even when it might have been erroring. This is technically a bug.

[bobzhang]

so I fixed that dry-run mode will not touch .compiler.log in #4710

[stale[bot]]

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