Add git reset --hard support to git module

Resets working tree to what is in HEAD and discards any
uncommitted changes.

library/git

@@ -90,6 +90,19 @@ def clone(repo, dest):
    cmd = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    return cmd.communicate()
 
+def reset(dest):
+   '''
+   Resets the index and working tree to HEAD.
+   Discards any changes to tracked files in working
+   tree since that commit.
+   '''
+   os.chdir(dest)
+   cmd = "git reset --hard HEAD"
+   cmd = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+   (out, err) = cmd.communicate()
+   rc = cmd.returncode
+   return (rc, out, err)
+
 def pull(repo, dest):
    ''' updates repo from remote sources '''
    os.chdir(dest)
@@ -123,6 +136,9 @@ if not os.path.exists(gitconfig):
 else:
    # else do a pull   
    before = get_version(dest)
+   (rc, out, err) = reset(dest)
+   if rc != 0:
+      fail_json(out=out, err=err)
    (out, err) = pull(repo, dest)
 
 # handle errors from clone or pull