Handle missing attribute files when parsing

Previously using Gitlab::Git::Attributes when $GIT_DIR/info/attributes
didn't exist would result in a runtime error. This commit fixes this so
an empty Hash is produced instead.

CHANGELOG

+v 10.6.1
+  - Fix parsing Git attributes when $GIT_DIR/info/attributes doesn't exist
+
 v 10.6.0
   - Use a pure Ruby Git attributes file parser to drastically reduce time spent in parsing Git attributes

lib/gitlab_git/attributes.rb

@@ -86,6 +86,8 @@ module Gitlab
       def each_line
         full_path = File.join(@path, 'info/attributes')
+        return unless File.exist?(full_path)
+
         File.open(full_path, 'r') do |handle|
           handle.each_line do |line|
             yield line.strip

spec/attributes_spec.rb

@@ -59,6 +59,14 @@ describe Gitlab::Git::Attributes do
     it 'ignores any comments and empty lines' do
       expect(subject.patterns.length).to eq(7)
     end
+
+    it 'does not parse anything when the attributes file does not exist' do
+      expect(File).to receive(:exist?).
+        with(File.join(path, 'info/attributes')).
+        and_return(false)
+
+      expect(subject.patterns).to eq({})
+    end
   end
   describe '#parse_attributes' do
@@ -93,5 +101,13 @@ describe Gitlab::Git::Attributes do
       expect { |b| subject.each_line(&b) }.to yield_successive_args(*args)
     end
+
+    it 'does not yield when the attributes file does not exist' do
+      expect(File).to receive(:exist?).
+        with(File.join(path, 'info/attributes')).
+        and_return(false)
+
+      expect { |b| subject.each_line(&b) }.not_to yield_control
+    end
   end
 end