This simplifies plot.py's -k/--keep-open logic into a self-contained
loop that just calls main_ on an update.
This is a compromise on getting rid of -k/--keep-open completely, since
we _could_ just rely on watch.py. But plot.py knowing which argument is
the file to watch is convenient.
The eventual plan is to adopt this small bit of copy-pastable-code in
the other ascii-art scripts (treemap.py, dbgbmap.py, etc).
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