Fix SystemStackError when Peek bar is active with Rugged calls

Peek attempts to serialize results with `to_json`, which calls
`ActiveSupport::JSON`. If an object is passed to `to_json` that contains
instance variables, `ActiveSupport` will attempt to recursively traverse
all variables.

The problem is that we can get into an infinite loop if the instance
references to an instance that references to something else that points
back to the same instance.

To avoid this mess, we just call `to_s` on the object. It appears only
`Gitlab::Git::Repository` and `::Repository` are the culprits here.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65404

lib/peek/views/rugged.rb

@@ -29,8 +29,12 @@ module Peek
 
       def format_args(args)
         args.map do |arg|
-          # Needed to avoid infinite as_json calls
-          if arg.is_a?(Gitlab::Git::Repository)
+          # ActiveSupport::JSON recursively calls as_json on all
+          # instance variables, and if that instance variable points to
+          # something that refers back to the same instance, we can wind
+          # up in an infinite loop. Currently this only seems to happen with
+          # Gitlab::Git::Repository and ::Repository.
+          if arg.instance_variables.present?
             arg.to_s
           else
             arg

spec/lib/peek/views/rugged_spec.rb

@@ -24,7 +24,7 @@ describe Peek::Views::Rugged, :request_store do
                                                      args: [project.repository.raw, 'HEAD'],
                                                      duration: 0.123)
     ::Gitlab::RuggedInstrumentation.add_call_details(feature: :rugged_test2,
-                                                     args: [project.repository.raw, 'refs/heads/master'],
+                                                     args: [project.repository, 'refs/heads/master'],
                                                      duration: 0.456)
 
     results = subject.results
@@ -32,7 +32,11 @@ describe Peek::Views::Rugged, :request_store do
     expect(results[:duration]).to eq('1234.00ms')
     expect(results[:details].count).to eq(2)
 
-    expect(results[:details][0][:args]).to eq([project.repository.raw.to_s, "refs/heads/master"])
-    expect(results[:details][1][:args]).to eq([project.repository.raw.to_s, "HEAD"])
+    expected = [
+      [project.repository.raw.to_s, "HEAD"],
+      [project.repository.to_s, "refs/heads/master"]
+    ]
+
+    expect(results[:details].map { |data| data[:args] }).to match_array(expected)
   end
 end