git: Return HEAD SHA after performing action

[benbroadaway]

Returns the latest HEAD ref of the branch an action is performed on. Example response for commitAndPush:

{
  "changeList" : [ "new-file.txt" ],
  "headSHA" : "95938259eadf6a32b7852bd230d8a8a70c5ef6da",
  "ok" : true,
  "error" : "",
  "status" : "SUCCESS"
}