fix: fetch at most 100 existing messages even if EXISTS was not received

According to RFC 3501, EXISTS must always be sent in response to SELECT.
But if the server does not send it for some reason,
async-imap uses the default value, so we will essentially fetch `1:*`
and downloading all messages may take a long time.

Author: link2xt

src/imap.rs

@@ -1324,8 +1324,8 @@ impl Imap {
         // Fetch last DC_FETCH_EXISTING_MSGS_COUNT (100) messages.
         // Sequence numbers are sequential. If there are 1000 messages in the inbox,
         // we can fetch the sequence numbers 900-1000 and get the last 100 messages.
-        let first = cmp::max(1, exists - DC_FETCH_EXISTING_MSGS_COUNT);
-        let set = format!("{first}:*");
+        let first = cmp::max(1, exists - DC_FETCH_EXISTING_MSGS_COUNT + 1);
+        let set = format!("{first}:{exists}");
         let mut list = session
             .fetch(&set, PREFETCH_FLAGS)
             .await