values given to make_log_request_dict are overridden

I believe that there's a bug in bzrlib/log.py, where the 'levels' value I pass to `make_log_request_dict` gets overridden by bzrlib.

The documentation suggests that when making my log request dictionary I should be able to specify 'levels=1' to limit the revisions printed by the Logger to mainline ones only.

However, what actually happens is that the value I specify gets overrwritten in bzrlib/log.py:357 with whatever the log formatter supports. The comment says:

# Tweak the LogRequest based on what the LogFormatter can handle.
# (There's no point generating stuff if the formatter can't display it.)

But the code will overwrite the levels value if the logger supports more levels than I wanted as well as fewer levels than I wanted!

Attached is a simple script that demonstrated the problem. It should print out only mainline revisions from a branch (that is specified as a first argument), but instead it prints out all levels.