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Minimize tempfile churn (#186)

Closes #185.

Try was making way too many tempfiles. Now we store everything in the sandbox, with a tiny bit of nuance: $IGNORE_FILE needs to be created in advance to handle the args properly. So we create that temporary unconditionally. When running a command (the try() function), we'll move $IGNORE_FILE into the sandbox. When running try commit or try summary, we just delete $IGNORE_FILE at the end.