[PATCH 2/6] am -3: support 3way merge on unborn branch

[Paul Tan <pyokagan@gmail.com>]

While on an unborn branch, git am -3 will fail to do a threeway merge as
it references HEAD as "our tree", but HEAD does not point to a valid
tree.

Fix this by using an empty tree as "our tree" when we are on an unborn
branch.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
---
 git-am.sh           | 3 ++-
 t/t4151-am-abort.sh | 9 +++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/git-am.sh b/git-am.sh
index df3c8f4..c847b58 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -179,7 +179,8 @@ It does not apply to blobs recorded in its index.")"
     then
 	    GIT_MERGE_VERBOSITY=0 && export GIT_MERGE_VERBOSITY
     fi
-    git-merge-recursive $orig_tree -- HEAD $his_tree || {
+    our_tree=$(git rev-parse --verify -q HEAD || echo $empty_tree)
+    git-merge-recursive $orig_tree -- $our_tree $his_tree || {
 	    git rerere $allow_rerere_autoupdate
 	    die "$(gettext "Failed to merge in the changes.")"
     }
diff --git a/t/t4151-am-abort.sh b/t/t4151-am-abort.sh
index e15acc8..b618ee0 100755
--- a/t/t4151-am-abort.sh
+++ b/t/t4151-am-abort.sh
@@ -83,4 +83,13 @@ test_expect_success 'am --abort will keep the local commits intact' '
 	test_cmp expect actual
 '
 
+test_expect_success 'am -3 stops on conflict on unborn branch' '
+	git checkout -f --orphan orphan &&
+	git reset &&
+	rm -f otherfile-4 &&
+	test_must_fail git am -3 0003-*.patch &&
+	test 2 -eq $(git ls-files -u | wc -l) &&
+	test 4 = "$(cat otherfile-4)"
+'
+
 test_done
-- 
2.1.4