## Changes
- Print warnings and errors by default.
- Fix ErrAlreadyPrinted not to be logged at Error level.
- Format log messages as "Warn: message" instead of "WARN" to make it
more readable and in-line with the rest of the output.
- Only print attributes (pid, mutator, etc) and time when the overall
level is debug (so --debug output has not changed much).
## Tests
- Existing acceptance tests show how warning messages appear in various
test case.
- Added new test for `--debug` output.
- Add sort_lines.py helper to avoid dependency on 'sort' which is
locale-sensitive.
## Changes
It wasn't working because it deferred to the regular `slog.TextHandler`
for the `WithAttr` and `WithGroup` functions. Both of these functions
don't mutate the handler but return a new one. When the top-level logger
called one of these, log records in that context used the standard
handler instead of ours.
To implement tracking of attributes and groups, I followed the guide at
https://github.com/golang/example/blob/master/slog-handler-guide/README.md
for writing custom handlers.
## Tests
The new tests demonstrate formatting through `t.Log` and look good.
Denis Bilenko
Pieter Noordhuis