am --skip: support skipping while on unborn branch

When git am --skip is run, git am will copy HEAD's tree entries to the
index with "git reset HEAD". However, on an unborn branch, HEAD does not
point to a tree, so "git reset HEAD" will fail.

Fix this by treating HEAD as en empty tree when we are on an unborn
branch.

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

Author: pyokagan

git-am.sh

@@ -499,9 +499,7 @@ then
 		git read-tree --reset -u $head_tree $head_tree &&
 		index_tree=$(git write-tree) &&
 		git read-tree -m -u $index_tree $head_tree
-		orig_head=$(cat "$GIT_DIR/ORIG_HEAD")
-		git reset HEAD
-		git update-ref ORIG_HEAD $orig_head
+		git read-tree $head_tree
 		;;
 	,t)
 		if test -f "$dotest/rebasing"

t/t4151-am-abort.sh

@@ -92,4 +92,14 @@ test_expect_success 'am -3 stops on conflict on unborn branch' '
 	test 4 = "$(cat otherfile-4)"
 '
 
+test_expect_success 'am -3 --skip clears index on unborn branch' '
+	test_path_is_dir .git/rebase-apply &&
+	echo tmpfile >tmpfile &&
+	git add tmpfile &&
+	git am --skip &&
+	test -z "$(git ls-files)" &&
+	test_path_is_missing otherfile-4 &&
+	test_path_is_missing tmpfile
+'
+
 test_done