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

am --skip: 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 succesfully merged files (such as a new
file). These changes to the index will not be reverted with a
"git read-tree --reset -u HEAD HEAD", as git read-tree will not be aware
of how the current index differs from HEAD.

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

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