Quote branch names in how to merge instructions

app/views/projects/merge_requests/_how_to_merge.html.haml

@@ -13,12 +13,13 @@
         %pre.dark#merge-info-1
           - if @merge_request.for_fork?
             :preserve
-              git fetch #{h default_url_to_repo(@merge_request.source_project)} #{h @merge_request.source_branch}
-              git checkout -b #{h @merge_request.source_project_path}-#{h @merge_request.source_branch} FETCH_HEAD
+              -# All repo/branch refs have been quoted to allow support for special characters (such as #my-branch)
+              git fetch "#{h default_url_to_repo(@merge_request.source_project)}" "#{h @merge_request.source_branch}"
+              git checkout -b "#{h @merge_request.source_project_path}-#{h @merge_request.source_branch}" FETCH_HEAD
           - else
             :preserve
               git fetch origin
-              git checkout -b #{h @merge_request.source_branch} origin/#{h @merge_request.source_branch}
+              git checkout -b "#{h @merge_request.source_branch}" "origin/#{h @merge_request.source_branch}"
         %p
           %strong Step 2.
           Review the changes locally
@@ -31,20 +32,20 @@
           - if @merge_request.for_fork?
             :preserve
               git fetch origin
-              git checkout origin/#{h @merge_request.target_branch}
-              git merge --no-ff #{h @merge_request.source_project_path}-#{h @merge_request.source_branch}
+              git checkout "origin/#{h @merge_request.target_branch}"
+              git merge --no-ff "#{h @merge_request.source_project_path}-#{h @merge_request.source_branch}"
           - else
             :preserve
               git fetch origin
-              git checkout origin/#{h @merge_request.target_branch}
-              git merge --no-ff #{h @merge_request.source_branch}
+              git checkout "origin/#{h @merge_request.target_branch}"
+              git merge --no-ff "#{h @merge_request.source_branch}"
         %p
           %strong Step 4.
           Push the result of the merge to GitLab
         = clipboard_button(target: "pre#merge-info-4", title: "Copy commands to clipboard")
         %pre.dark#merge-info-4
           :preserve
-            git push origin #{h @merge_request.target_branch}
+            git push origin "#{h @merge_request.target_branch}"
         - unless @merge_request.can_be_merged_by?(current_user)
           %p
             Note that pushing to GitLab requires write access to this repository.

changelogs/unreleased/quote-branch-names-in-instructions.yml

+---
+title: Quote branch names in how to merge instructions
+merge_request: 32639
+author: Lee Tickett
+type: fixed

spec/features/merge_request/user_selects_branches_for_new_mr_spec.rb

@@ -64,7 +64,7 @@ describe 'Merge request > User selects branches for new MR', :js do
 
     click_button "Check out branch"
 
-    expect(page).to have_content 'git checkout -b orphaned-branch origin/orphaned-branch'
+    expect(page).to have_content 'git checkout -b "orphaned-branch" "origin/orphaned-branch"'
   end
 
   it 'allows filtering multiple dropdowns' do