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>

1 files changed, 2 insertions, 1 deletions

diff --git a/git-am.sh b/git-am.sh
index 80a7fdbd24..c5e139ff33 100755
--- 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.")"
     }