getchar: Handle incomplete <Paste> in typeahead buffer #10311

<Paste> is a 3-byte sequence and the beginning one or two bytes can appear at
the very end of the typeahead buffer. When this happens, we were exiting from
`vgetorpeek()` instead of reading more characters to see the complete sequence.

I think this should fix #7994 -- at least partially. Before this change, when I
paste exactly 64 characters into a freshly booted instance, I get what I pasted
plus the literal text "<Paste>" at the end. Nvim also stays in nopaste mode.
The attached test case fails in this manner without the code change.

Fix #7994

src/nvim/getchar.c

@@ -1936,7 +1936,8 @@ static int vgetorpeek(int advance)
           }
 
           if ((mp == NULL || max_mlen >= mp_match_len)
-              && keylen != KEYLEN_PART_MAP) {
+              && keylen != KEYLEN_PART_MAP
+              && !(keylen == KEYLEN_PART_KEY && c1 == ui_toggle[0])) {
             // No matching mapping found or found a non-matching mapping that
             // matches at least what the matching mapping matched
             keylen = 0;