ui: add new config option for help text width Before this patch, when printing help text using `hg help`, or `hg log -h`, the output will wrap at 78 chars even if the user has a bigger terminal width and there is no config option to change it, making the experience different from the commonly used `man` tool. This patch introduces a new config option `ui.textwidth`, which replaces the hardcoded number. It's set to 78 by default to maintain compatibility. When set to 0, `hg help` will behave more like `man`.