Add whitespace filter to incremental mode

[Fix #441]

lib/mutant/repository.rb

@@ -41,6 +41,7 @@ module Mutant
         command = %W[
           git log
           #{from}...#{to}
+          --ignore-all-space
           -L #{line_range.begin},#{line_range.end}:#{path}
         ]
 

spec/unit/mutant/repository/diff_spec.rb

@@ -75,7 +75,7 @@ describe Mutant::Repository::Diff do
       end
 
       let(:expected_git_log_command) do
-        %W[git log from_rev...to_rev -L 1,2:#{path}]
+        %W[git log from_rev...to_rev --ignore-all-space -L 1,2:#{path}]
       end
 
       context 'on failure of git log command' do