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>

t/t2023-checkout-m.sh

@@ -46,4 +46,28 @@ test_expect_success '-m restores 3-way conflicted+resolved file' '
 	test_cmp both.txt.conflicted.cleaned both.txt.cleaned
 '
 
+test_expect_success 'force checkout a conflict file creates stage zero entry' '
+	git init co-force &&
+	(
+		cd co-force &&
+		echo a >a &&
+		git add a &&
+		git commit -ama &&
+		A_OBJ=$(git rev-parse :a) &&
+		git branch topic &&
+		echo b >a &&
+		git commit -amb &&
+		B_OBJ=$(git rev-parse :a) &&
+		git checkout topic &&
+		echo c >a &&
+		C_OBJ=$(git hash-object a) &&
+		git checkout -m master &&
+		test_cmp_rev :1:a $A_OBJ &&
+		test_cmp_rev :2:a $B_OBJ &&
+		test_cmp_rev :3:a $C_OBJ &&
+		git checkout -f topic &&
+		test_cmp_rev :0:a $A_OBJ
+	)
+'
+
 test_done

unpack-trees.c

@@ -2386,7 +2386,7 @@ int oneway_merge(const struct cache_entry * const *src,
 		if (o->update && S_ISGITLINK(old->ce_mode) &&
 		    should_update_submodules() && !verify_uptodate(old, o))
 			update |= CE_UPDATE;
-		add_entry(o, old, update, 0);
+		add_entry(o, old, update, CE_STAGEMASK);
 		return 0;
 	}
 	return merged_entry(a, old, o);