Tiny bug for log length not properly treated

Created by: blanet

A tiny issue for the logging part of syntect highlighting. It's obvious that firstCharacters uses hard-coded 10 instead of the passed in n.

https://github.com/sourcegraph/sourcegraph/blob/6029ee95960ded81f648dc1c24a637dbdb0a9bc9/internal/highlight/highlight.go#L239-L245