logging: Truncate long slow log messages

Created by: chrismwendt

The archive endpoint can get really long when many paths are specified:

/archive?path=foo&bar&baz&...

Combined with a buffering issue in sg, this was causing gitserver to hang when logging messages https://github.com/sourcegraph/sourcegraph/issues/30529#issuecomment-1029363414

After this change, long URLs will get truncated to something like /archive?path=foo&ba...(214398 more) for logging.