patch 9.0.0916: getbufline() is inefficient for getting a single line

Problem:    getbufline() is inefficient for getting a single line.
Solution:   Add getbufoneline().
diff --git a/src/testdir/test_bufline.vim b/src/testdir/test_bufline.vim
index 6021a4e..9e4402f 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -11,7 +11,9 @@
   hide
   call assert_equal(0, setbufline(b, 1, ['foo', 'bar']))
   call assert_equal(['foo'], getbufline(b, 1))
+  call assert_equal('foo', getbufoneline(b, 1))
   call assert_equal(['bar'], getbufline(b, '$'))
+  call assert_equal('bar', getbufoneline(b, '$'))
   call assert_equal(['foo', 'bar'], getbufline(b, 1, 2))
   exe "bd!" b
   call assert_equal([], getbufline(b, 1, 2))
@@ -35,8 +37,11 @@
 
   call assert_equal(0, setbufline(b, 4, ['d', 'e']))
   call assert_equal(['c'], b->getbufline(3))
+  call assert_equal('c', b->getbufoneline(3))
   call assert_equal(['d'], getbufline(b, 4))
+  call assert_equal('d', getbufoneline(b, 4))
   call assert_equal(['e'], getbufline(b, 5))
+  call assert_equal('e', getbufoneline(b, 5))
   call assert_equal([], getbufline(b, 6))
   call assert_equal([], getbufline(b, 2, 1))