[ruby/irb] Filter backtrace before format in handle_exception

(ruby/irb#916)

handle_exception now applies the filter_backtrace to exception
backtraces prior to formatting the lines with Exception#full_message

This fixes a bug in upstream projects, notably Rails, where the
backtrace filtering logic expects the lines to be formatted as
Exception#backtrace.

ruby/irb@805ee008f9

Co-authored-by: Hartley McGuire <skipkayhil@gmail.com>

lib/irb.rb

@@ -1222,6 +1222,13 @@ def handle_exception(exc)
         irb_bug = true
       else
         irb_bug = false
+        # This is mostly to make IRB work nicely with Rails console's backtrace filtering, which patches WorkSpace#filter_backtrace
+        # In such use case, we want to filter the exception's backtrace before its displayed through Exception#full_message
+        # And we clone the exception object in order to avoid mutating the original exception
+        # TODO: introduce better API to expose exception backtrace externally
+        backtrace = exc.backtrace.map { |l| @context.workspace.filter_backtrace(l) }.compact
+        exc = exc.clone
+        exc.set_backtrace(backtrace)
       end
 
       if RUBY_VERSION < '3.0.0'
@@ -1246,7 +1253,6 @@ def handle_exception(exc)
           lines = m.split("\n").reverse
         end
         unless irb_bug
-          lines = lines.map { |l| @context.workspace.filter_backtrace(l) }.compact
           if lines.size > @context.back_trace_limit
             omit = lines.size - @context.back_trace_limit
             lines = lines[0..(@context.back_trace_limit - 1)]