Handle custom scss-lint exit statuses

The `scss-lint` executable now returns a special status code when all
files that were specified were filtered by exclusions specified by its
configuration. Previously, this would return an error code that was
ambiguous, but it now returns a custom error code specifically for this
case.

Handling it in Overcommit allows us to save developers the hassle of
duplicating their exclusion configuration from `.scss-lint.yml` to
`.overcommit.yml`.

In the process, we also fixed the hook to always fail unless the status
code indicates lint warnings/errors, since that means the tool was
invoked incorrectly or some other problem is present.

Author: sds

lib/overcommit/hook/pre_commit/scss_lint.rb

@@ -7,7 +7,14 @@ class ScssLint < Base
 
     def run
       result = execute(command + applicable_files)
-      return :pass if result.success?
+
+      # Status code 81 indicates the applicable files were all filtered by
+      # exclusions defined by the configuration. In this case, we're happy to
+      # return success since there were technically no lints.
+      return :pass if [0, 81].include?(result.status)
+
+      # Any status that isn't indicating lint warnings or errors indicates failure
+      return :fail, result.stdout unless [1, 2].include?(result.status)
 
       extract_messages(
         result.stdout.split("\n"),

spec/overcommit/hook/pre_commit/scss_lint_spec.rb

@@ -12,7 +12,8 @@
   context 'when scss-lint exits successfully' do
     before do
       result = double('result')
-      result.stub(:success?).and_return(true)
+      result.stub(:status).and_return(0)
+      result.stub(:stdout).and_return('')
       subject.stub(:execute).and_return(result)
     end
 
@@ -23,12 +24,12 @@
     let(:result) { double('result') }
 
     before do
-      result.stub(:success?).and_return(false)
       subject.stub(:execute).and_return(result)
     end
 
     context 'and it reports a warning' do
       before do
+        result.stub(:status).and_return(1)
         result.stub(:stdout).and_return([
           'file1.scss:1 [W] Prefer single quoted strings',
         ].join("\n"))
@@ -39,12 +40,22 @@
 
     context 'and it reports an error' do
       before do
+        result.stub(:status).and_return(2)
         result.stub(:stdout).and_return([
           'file1.scss:1 [E] Syntax Error: Invalid CSS',
         ].join("\n"))
       end
 
       it { should fail_hook }
     end
+
+    context 'and it returns status code indicating all files were filtered' do
+      before do
+        result.stub(:status).and_return(81)
+        result.stub(:stdout).and_return('')
+      end
+
+      it { should pass }
+    end
   end
 end