unpack-trees: fix oneway_merge accidentally carry over stage index

unpack-trees: fix oneway_merge accidentally carry over stage index

Phillip found out that 'git checkout -f <branch>' does not restore
conflict/unmerged files correctly. All tracked files should be taken
from <branch> and all non-zero stages removed. Most of this is true,
except that the final file could be in stage one instead of zero.

"checkout -f" (among other commands) does this with one-way merge, which
is supposed to take stat info from the index and everything else from
the given tree. The add_entry(.., old, ...) call in oneway_merge()
though will keep stage index from the index.

This is normally not a problem if the entry from the index is
normal (stage #0). But if there is a conflict, stage #0 does not exist
and we'll get stage #1 entry as "old" variable, which gets recorded in
the final index. Fix it by clearing stage mask.

This bug probably comes from b5b425074e (git-read-tree: make one-way
merge also honor the "update" flag, 2005-06-07). Before this commit, we
may create the final ("dst") index entry from the one in index, but we
do clear CE_STAGEMASK.

I briefly checked two- and three-way merge functions. I think we don't
have the same problem in those.

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>