am -3: support 3way merge on unborn branch

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-am.sh b/git-am.sh
index 80a7fdb..c5e139f 100755 (executable)
--- a/git-am.sh
+++ b/git-am.sh
@@ -178,7 +178,8 @@ 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 5ac741a..2683ffa 100755 (executable)
--- 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