cli --ignore-immutable help: remove double negative

cli/src/cli_util.rs

@@ -2366,7 +2366,7 @@ pub struct GlobalArgs {
     /// implies `--ignore-working-copy`.
     #[arg(long, global = true)]
     pub ignore_working_copy: bool,
-    /// Don't prevent rewriting immutable commits
+    /// Allow rewriting immutable commits
     ///
     /// By default, Jujutsu prevents rewriting commits in the configured set of
     /// immutable commits. This option disables that check and lets you rewrite

cli/tests/cli-reference@.md.snap

@@ -153,7 +153,7 @@ To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/d
 
   Possible values: `true`, `false`
 
-* `--ignore-immutable` — Don't prevent rewriting immutable commits
+* `--ignore-immutable` — Allow rewriting immutable commits
 
   Possible values: `true`, `false`
 

cli/tests/test_global_opts.rs

@@ -592,7 +592,7 @@ fn test_help() {
     Global Options:
       -R, --repository <REPOSITORY>      Path to repository to operate on
           --ignore-working-copy          Don't snapshot the working copy, and don't update it
-          --ignore-immutable             Don't prevent rewriting immutable commits
+          --ignore-immutable             Allow rewriting immutable commits
           --at-operation <AT_OPERATION>  Operation to load the repo at [default: @] [aliases: at-op]
           --debug                        Enable debug logging
           --color <WHEN>                 When to colorize output (always, never, auto)