Everything from gitlab_git is already UTF-8.

app/controllers/projects/graphs_controller.rb

@@ -28,8 +28,8 @@ class Projects::GraphsController < Projects::ApplicationController
 
     @commits.each do |commit|
       @log << {
-        author_name: commit.author_name.force_encoding('UTF-8'),
-        author_email: commit.author_email.force_encoding('UTF-8'),
+        author_name: commit.author_name,
+        author_email: commit.author_email,
         date: commit.committed_date.strftime("%Y-%m-%d")
       }
     end

app/models/repository.rb

@@ -136,8 +136,8 @@ class Repository
         commit = Gitlab::Git::Commit.new(rugged_commit)
 
         {
-          author_name: commit.author_name.force_encoding('UTF-8'),
-          author_email: commit.author_email.force_encoding('UTF-8'),
+          author_name: commit.author_name,
+          author_email: commit.author_email,
           additions: commit.stats.additions,
           deletions: commit.stats.deletions,
         }

app/views/projects/blame/show.html.haml

@@ -30,5 +30,5 @@
                 %code
                   :erb
                     <% lines.each do |line| %>
-                      <%= highlight(@blob.name, line.force_encoding("utf-8"), true).html_safe %>
+                      <%= highlight(@blob.name, line, true).html_safe %>
                     <% end %>