Fix following with multiple paths

`git log --follow` is only supported for a single path. CE doesn't currently
pass multiple paths, but EE does.

app/models/repository.rb

@@ -109,7 +109,7 @@ class Repository
       offset: offset,
       after: after,
       before: before,
-      follow: path.present?,
+      follow: Array(path).length == 1,
       skip_merges: skip_merges
     }
 

spec/models/repository_spec.rb

@@ -171,6 +171,27 @@ describe Repository, models: true do
     end
   end
 
+  describe '#commits' do
+    it 'sets follow when path is a single path' do
+      expect(Gitlab::Git::Commit).to receive(:where).with(a_hash_including(follow: true)).and_call_original.twice
+
+      repository.commits('master', path: 'README.md')
+      repository.commits('master', path: ['README.md'])
+    end
+
+    it 'does not set follow when path is multiple paths' do
+      expect(Gitlab::Git::Commit).to receive(:where).with(a_hash_including(follow: false)).and_call_original
+
+      repository.commits('master', path: ['README.md', 'CHANGELOG'])
+    end
+
+    it 'does not set follow when there are no paths' do
+      expect(Gitlab::Git::Commit).to receive(:where).with(a_hash_including(follow: false)).and_call_original
+
+      repository.commits('master')
+    end
+  end
+
   describe '#find_commits_by_message' do
     it 'returns commits with messages containing a given string' do
       commit_ids = repository.find_commits_by_message('submodule').map(&:id)