merge-one-file: leave unmerged index entries upon automerge failure.

When automerge fails, we used to collapse the path to stage0
from "our" branch, to help "diff-files" users to view the
half-merged state against the current HEAD.  Now diff-files has
been taught how to compare with unmerged stage2,leaving them
unmerged is a better thing to do, especially this prevents the
unresolved conflicts to be committed by mistake.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge-one-file.sh

@@ -79,11 +79,7 @@ case "${1:-.}${2:-.}${3:-.}" in
 		;;
 	esac
 
-	# We reset the index to the first branch, making
-	# git-diff-file useful
-	git-update-index --add --cacheinfo "$6" "$2" "$4"
-		git-checkout-index -u -f -- "$4" &&
-		merge "$4" "$orig" "$src2"
+	merge "$4" "$orig" "$src2"
 	ret=$?
 	rm -f -- "$orig" "$src2"