pager: default to less -FRX

As dbarnett@ reported on #9, our default of `less`, combined with our
default of enabling color on TTYs, means that we print ANSI codes to
`less` by default. Unless the user has set e.g. `$LESS=R`, `less` is
going to escape those codes, resulting in garbage like this:

```
@ ESC[1;35mbb39c26a29feESC[0m ESC[1;33m(no email configured)ESC[0m ESC[1;36m2022-12-03....
```

I guess most of us didn't notice because we have something like
`$LESS=FRX` set.

This patch changes our default from `less` to `less -FRX`. Those are
the flags we're using for our internal hg distribution at Google, and
that has seemed quite uncontroversial.

I added a pointer from the changelog to the tracking issue while at
it.

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### New features
 
 * Commands with long output are paginated.
+  [#9](https://github.com/martinvonz/jj/issues/9)
 
 * The new `jj git remote rename` command allows git remotes to be renamed
   in-place.

src/ui.rs

@@ -111,7 +111,7 @@ fn pager_setting(settings: &UserSettings) -> FullCommandArgs {
     settings
         .config()
         .get("ui.pager")
-        .unwrap_or_else(|_| "less".into())
+        .unwrap_or_else(|_| "less -FRX".into())
 }
 
 impl Ui {