Change UTF-8 combining to inspect the previous character at the cursor

position rather than keeping the last character from the input stream,
this is how most terminals work and fixes problems with displaying these
characters in vim. GitHub issue 3600.

tty.c

@@ -2091,6 +2091,9 @@ tty_cmd_cell(struct tty *tty, const struct tty_ctx *ctx)
 
 	tty_cell(tty, ctx->cell, &ctx->defaults, ctx->palette,
 	    ctx->s->hyperlinks);
+
+	if (ctx->num == 1)
+		tty_invalidate(tty);
 }
 
 void