Bugfix the new pull --clobber to not generate spurious conflicts.
When --clobber clobbered the history, a bad merge base was used.
Supporting this: * merge.merge_inner now has tempdir as an optional parameter. (Robert Collins)
* Tree.kind is not recorded at the top level of the hierarchy, as it was missing on EmptyTree, leading to a bug with merge on EmptyTrees. (Robert Collins)