am --abort: revert changes introduced by failed 3way merge

Even when a merge conflict occurs with am --3way, the index will be
modified with the results of any successfully merged files. These
changes to the index will not be reverted with a
"git read-tree --reset -u HEAD ORIG_HEAD", as git read-tree will not be
aware of how the current index differs from HEAD or ORIG_HEAD.

To fix this, we first reset any conflicting entries in the index. The
resulting index will contain the results of successfully merged files
introduced by the failed merge. We write this index to a tree, and then
use git read-tree to fast-forward this "index tree" back to ORIG_HEAD,
thus undoing all the changes from the failed merge.

When we are on an unborn branch, HEAD and ORIG_HEAD will not point to
valid trees. In this case, use an empty tree.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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