bf7ecd6892 went too far and broke
importing, the old tree was used on the remote tracking branch and not
the newly imported tree.
Test suite noticed the problem luckily.
Fix reversion in last release that caused wrong tree to be written to
remote tracking branch after an export of a subtree.
The invariant "commitsha should have the treesha as its tree"
was not met due to a bug. Guarantee it's met by catting the commitsha
to find its actual tree. A little bit slower, but this is not run often.
