Use git to check for empty index.

spec/test_spec.rb

@@ -57,7 +57,7 @@ describe FormatStaged do
     expect(repo.get_content('test.other')).to eq('x=y')
   end
 
-  it 'fails if files are changed to already comitted version' do
+  it 'fails if all files are changed to already comitted version' do
     repo.file_in_tree 'test.test', 'x = y'
     repo.set_content 'test.test', 'x=y'
     repo.stage 'test.test'
@@ -68,7 +68,55 @@ describe FormatStaged do
     expect(repo.get_content('test.test')).to eq('x = y')
   end
 
+  it 'succeeds if there are excluded files to commit' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', 'x=y'
+    repo.stage 'test.test'
+    repo.set_content 'test.other', 'abc'
+    repo.stage 'test.other'
+
+    success = repo.run_formatter
+
+    expect(success).to be_truthy
+    expect(repo.get_content('test.test')).to eq('x = y')
+  end
+
   it 'succeeds if there are no staged files' do
     expect(repo.run_formatter).to be_truthy
   end
+
+  it 'succeeds if only excluded files are changed' do
+    repo.set_content 'test.other', 'abc'
+    repo.stage 'test.other'
+
+    expect(repo.run_formatter).to be_truthy
+  end
+
+  it 'succeeds if one file is changed' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', 'x=y'
+    repo.stage 'test.test'
+    repo.set_content 'other.test', 'a=b'
+    repo.stage 'other.test'
+
+    success = repo.run_formatter
+
+    expect(success).to be_truthy
+    expect(repo.get_content('test.test')).to eq('x = y')
+    expect(repo.get_content('other.test')).to eq('a = b')
+  end
+
+  it 'fails if a single file becomes empty' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', '#clear'
+    repo.stage 'test.test'
+    repo.set_content 'other.test', 'a=b'
+    repo.stage 'other.test'
+
+    success = repo.run_formatter
+
+    expect(success).to be_falsy
+    expect(repo.get_content('test.test')).to eq('#clear')
+    expect(repo.get_content('other.test')).to eq('a = b')
+  end
 end