am --abort: support aborting to unborn branch

When git-am is first run on an unborn branch, no ORIG_HEAD is created.
As such, any applied commits will remain even after a git am --abort.

To be consistent with the behavior of git am --abort when it is not run
from an unborn branch, we empty the index, and then destroy the branch
pointed to by HEAD if there is no ORIG_HEAD.

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

Author: pyokagan

git-am.sh

@@ -514,7 +514,14 @@ then
 			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
+			if git rev-parse --verify -q ORIG_HEAD >/dev/null 2>&1
+			then
+				git reset ORIG_HEAD
+			else
+				git read-tree $empty_tree
+				curr_branch=$(git symbolic-ref HEAD 2>/dev/null) &&
+				git update-ref -d $curr_branch
+			fi
 		fi
 		rm -fr "$dotest"
 		exit ;;

t/t4151-am-abort.sh

@@ -14,6 +14,7 @@ test_expect_success setup '
 	git add file-1 file-2 &&
 	git commit -m initial &&
 	git tag initial &&
+	git format-patch --stdout --root initial >initial.patch &&
 	for i in 2 3 4 5 6
 	do
 		echo $i >>file-1 &&
@@ -125,4 +126,20 @@ test_expect_success 'am -3 --abort removes otherfile-4 on unborn branch' '
 	test_path_is_missing otherfile-4
 '
 
+test_expect_success 'am -3 --abort on unborn branch removes applied commits' '
+	git checkout -f --orphan orphan &&
+	git reset &&
+	rm -f otherfile-4 otherfile-2 file-1 file-2 &&
+	test_must_fail git am -3 initial.patch 0003-*.patch &&
+	test 3 -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_path_is_missing file-1 &&
+	test_path_is_missing file-2 &&
+	test 0 -eq $(git log --oneline 2>/dev/null | wc -l) &&
+	test refs/heads/orphan = "$(git symbolic-ref HEAD)"
+'
+
 test_done