lsp: use getbufoneline when possible

use getbufoneline when it's available to get a line instead of getting
all the lines of a buffer and selecting only the line of interest.

Author: bhcleek

autoload/go/lsp.vim

@@ -2048,7 +2048,11 @@ function! s:lineinfile(fname, line) abort
     if l:bufnr == -1 || len(l:bufinfo) == 0 || l:bufinfo[0].loaded == 0
       let l:filecontents = readfile(a:fname, '', a:line)
     else
-      let l:filecontents = getbufline(a:fname, a:line)
+      if exists('*getbufoneline')
+        let l:filecontents = [getbufoneline(a:fname, a:line)]
+      else
+        let l:filecontents = getbufline(a:fname, a:line)
+      endif
     endif
 
     if len(l:filecontents) == 0