vim-patch:8.2.5058: input() does not handle composing characters properly

Problem:    input() does not handle composing characters properly.
Solution:   Use mb_cptr2char_adv() instead of mb_ptr2char_adv().
            (closes vim/vim#10527)
e3a529bc87

Cherry-pick all of Test_input_func() from patch 8.2.0316.

(cherry picked from commit 84ed71f319)

src/nvim/getchar.c

@@ -631,7 +631,7 @@ void stuffReadbuffSpec(const char *s)
       stuffReadbuffLen(s, 3);
       s += 3;
     } else {
-      int c = mb_ptr2char_adv((const char_u **)&s);
+      int c = mb_cptr2char_adv((const char_u **)&s);
       if (c == CAR || c == NL || c == ESC) {
         c = ' ';
       }

src/nvim/testdir/test_functions.vim

@@ -1180,6 +1180,45 @@ func Test_col()
   bw!
 endfunc
 
+" Test for input()
+func Test_input_func()
+  " Test for prompt with multiple lines
+  redir => v
+  call feedkeys(":let c = input(\"A\\nB\\nC\\n? \")\<CR>B\<CR>", 'xt')
+  redir END
+  call assert_equal("B", c)
+  call assert_equal(['A', 'B', 'C'], split(v, "\n"))
+
+  " Test for default value
+  call feedkeys(":let c = input('color? ', 'red')\<CR>\<CR>", 'xt')
+  call assert_equal('red', c)
+
+  " Test for completion at the input prompt
+  func! Tcomplete(arglead, cmdline, pos)
+    return "item1\nitem2\nitem3"
+  endfunc
+  call feedkeys(":let c = input('Q? ', '', 'custom,Tcomplete')\<CR>"
+        \ .. "\<C-A>\<CR>", 'xt')
+  delfunc Tcomplete
+  call assert_equal('item1 item2 item3', c)
+
+  " Test for using special characters as default input
+  call feedkeys(":let c = input('name? ', \"x\\<BS>y\")\<CR>\<CR>", 'xt')
+  call assert_equal('y', c)
+
+  " Test for using text with composing characters as default input
+  call feedkeys(":let c = input('name? ', \"ã̳\")\<CR>\<CR>", 'xt')
+  call assert_equal('ã̳', c)
+
+  " Test for using <CR> as default input
+  call feedkeys(":let c = input('name? ', \"\\<CR>\")\<CR>x\<CR>", 'xt')
+  call assert_equal(' x', c)
+
+  call assert_fails("call input('F:', '', 'invalid')", 'E180:')
+  call assert_fails("call input('F:', '', [])", 'E730:')
+endfunc
+
+" Test for inputlist()
 func Test_inputlist()
   call feedkeys(":let c = inputlist(['Select color:', '1. red', '2. green', '3. blue'])\<cr>1\<cr>", 'tx')
   call assert_equal(1, c)