don't try to reassign a reviewer when a pr title is edited

doing so leads to weird errors:

> Could not assign reviewer from: jyn514.
> User(s) jyn514 are either the PR author, already assigned, or on vacation, and there are no other candidates.
> Use r? to specify someone else to assign.

this was likely missed when transitioning from highfive to triagebot. `handle_command` is run *both*
when the PR is opened and when it is edited (see the `command_handlers` macro). but in both those
cases `handle_input` will also run. don't override its decisions.

(`handle_input` already handles edits correctly, by not setting an assignee if one already exists.)

Author: jyn514

src/handlers/assign.rs

@@ -478,7 +478,7 @@ pub(super) async fn handle_command(
                 if matches!(
                     event,
                     Event::Issue(IssuesEvent {
-                        action: IssuesAction::Opened,
+                        action: IssuesAction::Opened | IssuesAction::Edited,
                         ..
                     })
                 ) {