Allow use of system git for git fetch if USE_SYSTEM_GIT_FOR_FETCH is defined

This is a workaround until we can ship git 2.16 with GitLab.

Closes #5214

ee/changelogs/unreleased/sh-allow-system-git-use-env.yml

+---
+title: Allow use of system git for git fetch if USE_SYSTEM_GIT_FOR_FETCH is defined
+merge_request:
+author:
+type: other

lib/gitlab/git/gitlab_projects.rb

@@ -67,7 +67,7 @@ module Gitlab
         tags_option = tags ? '--tags' : '--no-tags'
 
         logger.info "Fetching remote #{name} for repository #{repository_absolute_path}."
-        cmd = %W(git fetch #{name} --quiet)
+        cmd = %W(#{git_fetch_binary} fetch #{name} --quiet)
         cmd << '--prune' if prune
         cmd << '--force' if force
         cmd << tags_option
@@ -83,6 +83,14 @@ module Gitlab
         end
       end
 
+      # This is a workaround for Geo until we can ship git 2.16
+      # See https://gitlab.com/gitlab-org/gitlab-ee/issues/5214
+      def git_fetch_binary
+        return 'git' unless ENV['USE_SYSTEM_GIT_FOR_FETCH'] == "1"
+
+        '/usr/bin/git'
+      end
+
       def push_branches(remote_name, timeout, force, branch_names)
         logger.info "Pushing branches from #{repository_absolute_path} to remote #{remote_name}: #{branch_names}"
         cmd = %w(git push)

spec/lib/gitlab/git/gitlab_projects_spec.rb

@@ -160,6 +160,20 @@ describe Gitlab::Git::GitlabProjects do
         expect(key.string).to eq('KNOWN HOSTS')
       end
     end
+
+    context 'with USE_SYSTEM_GIT_FOR_FETCH env' do
+      let(:cmd) { %W(/usr/bin/git fetch #{remote_name} --quiet --prune --tags) }
+
+      before do
+        stub_env('USE_SYSTEM_GIT_FOR_FETCH', '1')
+      end
+
+      it 'uses system git' do
+        stub_spawn(cmd, 600, tmp_repo_path, {}, success: true)
+
+        is_expected.to be_truthy
+      end
+    end
   end
 
   describe '#import_project' do