Bug in subtree merge and checkout.

[bryanhirsch]

I'm doing some tests with this example site at https://github.com/DrupalLadder/_rhymessite/commit/a78f0af7c95d0b78a742eeb0fabad9809124e383:

These fail:

drush subtree checkout rhymes 7.x-1.0
drush subtree-compare-tags

drush subtree merge rhymes # defaults to 7.x-1.0
drush subtree-compare-tags

This passes:

drush buildmanager-build # merges to rhymes 7.x-1.0
drush subtree-compare-tags