'/usr/bin/tahoe debug --help' gives incorrect information about running coverage on the installed Debian/Ubuntu Tahoe

[davidsarah]

If current Tahoe were packaged for Debian/Ubuntu, the following information printed by /usr/bin/tahoe debug --help would be wrong:

Another debugging feature is that bin/tahoe allows executing an arbitrary
"runner" command (typically an installed Python script, such as 'coverage'),
with the Tahoe libraries on the PYTHONPATH. The runner command name is
prefixed with '@', and any occurrences of '@tahoe' in its arguments are
replaced by the full path to the tahoe script.

For example, if 'coverage' is installed and on the PATH, you can use:

    bin/tahoe @coverage run --branch @tahoe debug trial

to get branch coverage for the Tahoe test suite. Or, to run python with
the -3 option that warns about Python 3 incompatibilities:

    bin/tahoe @python -3 @tahoe command [options]

The described feature added in #1303 is not supported by the /usr/bin/tahoe script (and isn't needed, because it doesn't indirect through a subprocess). The correct information is:

To get branch coverage for the Tahoe test suite (on the installed copy of
Tahoe), install the 'python-coverage' package and then use:

    python-coverage run --branch /usr/bin/tahoe debug trial