run: Fix up various things.

Adress the post-merge comments from #2486, which found a doc comment
inaccurate and to not blindly ignore `-j 0` which would've worked until now.
I've also reduced the default `jobs` size to one, as it's user-visible configuration
which determines how many processes should run.

Thanks to @necauqua the controversial `unsafe` usage was already removed.

I've omitted to change `revisions` from an Vec to a RevisonArg for the moment, 
as I will keep working on the file anyway.

cli/src/commands/run.rs

@@ -39,8 +39,6 @@ pub struct RunArgs {
     /// The command to run across all selected revisions.
     shell_command: String,
     /// The revisions to change.
-    /// Multiple revsets are accepted and the work will be done on a
-    /// intersection of them.
     #[arg(long, short, default_value = "@")]
     revisions: Vec<RevisionArg>,
     /// A no-op option to match the interface of `git rebase -x`.
@@ -55,16 +53,16 @@ pub fn cmd_run(ui: &mut Ui, command: &CommandHelper, args: &RunArgs) -> Result<(
     let workspace_command = command.workspace_helper(ui)?;
     let _resolved_commits =
         resolve_multiple_nonempty_revsets(&args.revisions, &workspace_command, ui)?;
-    let _jobs = if let Some(_jobs) = args.jobs {
-        _jobs
-    } else {
-        // Use all available cores
-
-        // SAFETY:
-        // We use a internal constant of 4 threads, if it fails
-        let available =
-            std::thread::available_parallelism().unwrap_or(NonZeroUsize::new(4).unwrap());
-        available.into()
-    };
+    let _jobs = args
+        .jobs
+        .filter(|j| *j > 0)
+        .or_else(|| {
+            // Use all available cores
+            std::thread::available_parallelism()
+                .ok()
+                .and_then(|t| t.try_into().ok())
+        })
+        // Fallback to a single user-visible job.
+        .unwrap_or(1usize);
     Err(user_error("This is a stub, do not use"))
 }