fix(git_changelog_generate): pass tag if it exists to gitReadLog

Previously if a tag was found the script would try to find commits
between undefined..HEAD. By passing the tag, it now finds tags between
tag..HEAD.

Closes rafinskipg#5.

tasks/git_changelog_generate.js

@@ -249,7 +249,7 @@ var generate = function(params) {
 
  if(typeof(tag) !== 'undefined'){
  console.log('Reading git log since', tag);
- fn = function(){ return readGitLog(GIT_LOG_CMD);};
+ fn = function(){ return readGitLog(GIT_LOG_CMD, tag);};
  }else{
  console.log('Reading git log since the beggining');
  fn = function(){ return readGitLog(GIT_NOTAG_LOG_CMD);};