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>

git-am.sh

@@ -509,7 +509,11 @@ then
 		git rerere clear
 		if safe_to_abort
 		then
-			git read-tree --reset -u HEAD ORIG_HEAD
+			head_tree=$(git rev-parse --verify -q HEAD || echo $empty_tree) &&
+			git read-tree --reset -u $head_tree $head_tree &&
+			index_tree=$(git write-tree) &&
+			orig_head=$(git rev-parse --verify -q ORIG_HEAD || echo $empty_tree) &&
+			git read-tree -m -u $index_tree $orig_head
 			git reset ORIG_HEAD
 		fi
 		rm -fr "$dotest"

t/t4151-am-abort.sh

@@ -70,6 +70,17 @@ test_expect_success 'am -3 --skip removes otherfile-4' '
 	test 4 = "$(cat otherfile-4)" &&
 	git am --skip &&
 	test_cmp_rev initial HEAD &&
+	test -z "$(git ls-files -u)" &&
+	test_path_is_missing otherfile-4
+'
+
+test_expect_success 'am -3 --abort removes otherfile-4' '
+	git reset --hard initial &&
+	test_must_fail git am -3 0003-*.patch &&
+	test 3 -eq $(git ls-files -u | wc -l) &&
+	test 4 = "$(cat otherfile-4)" &&
+	git am --abort &&
+	test_cmp_rev initial HEAD &&
 	test -z $(git ls-files -u) &&
 	test_path_is_missing otherfile-4
 '
@@ -102,4 +113,16 @@ test_expect_success 'am -3 --skip clears index on unborn branch' '
 	test_path_is_missing tmpfile
 '
 
+test_expect_success 'am -3 --abort removes otherfile-4 on unborn branch' '
+	git checkout -f --orphan orphan &&
+	git reset &&
+	rm -f otherfile-4 file-1 &&
+	test_must_fail git am -3 0003-*.patch &&
+	test 2 -eq $(git ls-files -u | wc -l) &&
+	test 4 = "$(cat otherfile-4)" &&
+	git am --abort &&
+	test -z "$(git ls-files -u)" &&
+	test_path_is_missing otherfile-4
+'
+
 test_done