Fix flushing of commits to always force flush the last batch

Fix:
 * Flushing commits to take a 'force' param to force the last batch of
   commits to be flushed (the last set was not being saved to JSON)
 * A shadowing issue of the variable 'file' in commits.rb

lib/git_statistics/collector.rb

@@ -19,6 +19,7 @@ def collect(branch, options = {})
         extract_commit(commit)
         @commits.flush_commits
       end
+      @commits.flush_commits(true)
     end
 
     def acquire_commit_meta(commit_summary)

lib/git_statistics/commits.rb

@@ -33,8 +33,8 @@ def files_in_path
       Dir.entries(path).reject { |file| %w[. ..].include?(file) }
     end
 
-    def flush_commits
-      if size >= limit
+    def flush_commits(force=false)
+      if size >= limit || force
         file_count = Utilities.number_of_matching_files(path, /\d+\.json/)
         save(File.join(path, "#{file_count}.json"), @pretty)
         clear
@@ -145,9 +145,9 @@ def save(file, pretty)
         # Ensure the path to the file exists
         FileUtils.mkdir_p(File.dirname(file))
         # Save file in a simple or pretty format
-        File.open(file, 'w') do |file|
+        File.open(file, 'w') do |f|
           json_content = pretty ? JSON.pretty_generate(self) : self.to_json
-          file.write(json_content)
+          f.write(json_content)
         end
       end
     end