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))
 
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index ecbfeac..005f89c 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1724,15 +1724,23 @@
   getbufline(-1, '$', '$')->assert_equal([])
   getbufline(-1, 1, '$')->assert_equal([])
 
+  getbufoneline('#', 1)->assert_equal(lines[0])
+
   assert_equal([7, 7, 7], getbufline('#', 1, '$')->map((_, _) => 7))
 
   assert_fails('getbufline("", "$a", "$b")', ['E1030: Using a String as a Number: "$a"', 'E1030: Using a String as a Number: "$a"'])
   assert_fails('getbufline("", "$", "$b")', ['E1030: Using a String as a Number: "$b"', 'E1030: Using a String as a Number: "$b"'])
   bwipe!
 
+  assert_fails('getbufoneline("", "$a")', ['E1030: Using a String as a Number: "$a"', 'E1030: Using a String as a Number: "$a"'])
+  bwipe!
+
   v9.CheckDefAndScriptFailure(['getbufline([], 2)'], ['E1013: Argument 1: type mismatch, expected string but got list<unknown>', 'E1220: String or Number required for argument 1'])
   v9.CheckDefAndScriptFailure(['getbufline("a", [])'], ['E1013: Argument 2: type mismatch, expected string but got list<unknown>', 'E1220: String or Number required for argument 2'])
   v9.CheckDefAndScriptFailure(['getbufline("a", 2, 0z10)'], ['E1013: Argument 3: type mismatch, expected string but got blob', 'E1220: String or Number required for argument 3'])
+
+  v9.CheckDefAndScriptFailure(['getbufoneline([], 2)'], ['E1013: Argument 1: type mismatch, expected string but got list<unknown>', 'E1220: String or Number required for argument 1'])
+  v9.CheckDefAndScriptFailure(['getbufoneline("a", [])'], ['E1013: Argument 2: type mismatch, expected string but got list<unknown>', 'E1220: String or Number required for argument 2'])
 enddef
 
 def Test_getbufvar()