Fix a rather longstanding corner-case bug in twoway "reset to
there" merge, which is most often seen in "git am --abort".

* jk/two-way-merge-corner-case-fix:
  t1005: add test for "read-tree --reset -u A B"
  t1005: reindent
  unpack-trees: fix "read-tree -u --reset A B" with conflicted index