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/evalfunc.c b/src/evalfunc.c
index 310fa78..e4f28ed 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1923,6 +1923,8 @@
 			ret_list_dict_any,  f_getbufinfo},
     {"getbufline",	2, 3, FEARG_1,	    arg3_buffer_lnum_lnum,
 			ret_list_string,    f_getbufline},
+    {"getbufoneline",	2, 2, FEARG_1,	    arg2_buffer_lnum,
+			ret_string,	    f_getbufoneline},
     {"getbufvar",	2, 3, FEARG_1,	    arg3_buffer_string_any,
 			ret_any,	    f_getbufvar},
     {"getchangelist",	0, 1, FEARG_1,	    arg1_buffer,