Merge branch 'jk/pull-into-dirty-unborn' into maint

"git pull" into nothing trashed "local changes" that were in the
index.

* jk/pull-into-dirty-unborn:
  pull: merge into unborn by fast-forwarding from empty tree
  pull: update unborn branch tip after index

git-pull.sh

@@ -266,10 +266,17 @@ case "$merge_head" in
 	;;
 esac
 
+# Pulling into unborn branch: a shorthand for branching off
+# FETCH_HEAD, for lazy typers.
 if test -z "$orig_head"
 then
-	git update-ref -m "initial pull" HEAD $merge_head "$curr_head" &&
-	git read-tree -m -u HEAD || exit 1
+	# Two-way merge: we claim the index is based on an empty tree,
+	# and try to fast-forward to HEAD.  This ensures we will not
+	# lose index/worktree changes that the user already made on
+	# the unborn branch.
+	empty_tree=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+	git read-tree -m -u $empty_tree $merge_head &&
+	git update-ref -m "initial pull" HEAD $merge_head "$curr_head"
 	exit
 fi
 

t/t5520-pull.sh

@@ -57,6 +57,35 @@ test_expect_success 'pulling into void does not overwrite untracked files' '
 	)
 '
 
+test_expect_success 'pulling into void does not overwrite staged files' '
+	git init cloned-staged-colliding &&
+	(
+		cd cloned-staged-colliding &&
+		echo "alternate content" >file &&
+		git add file &&
+		test_must_fail git pull .. master &&
+		echo "alternate content" >expect &&
+		test_cmp expect file &&
+		git cat-file blob :file >file.index &&
+		test_cmp expect file.index
+	)
+'
+
+
+test_expect_success 'pulling into void does not remove new staged files' '
+	git init cloned-staged-new &&
+	(
+		cd cloned-staged-new &&
+		echo "new tracked file" >newfile &&
+		git add newfile &&
+		git pull .. master &&
+		echo "new tracked file" >expect &&
+		test_cmp expect newfile &&
+		git cat-file blob :newfile >newfile.index &&
+		test_cmp expect newfile.index
+	)
+'
+
 test_expect_success 'test . as a remote' '
 
 	git branch copy master &&