Don't specify the password on the command line

https://www.netmeister.org/blog/passing-passwords.html has a decent
overview of the security issues with that.

Since this is interactive, getting curl to prompt for the password is
probably the easiest alternative. From
https://curl.se/docs/manpage.html#-u

> If you simply specify the username, curl prompts for a password.

I don't have an easy way to test the actual command in this doc, but the
flag does seem to behave as documented at least:

```
$ curl --user user@example.com https://google.com
Enter host password for user 'user@example.com'
```

Fixes gitgitgadget/gitgitgadget#1843

Author: dseomn

reply-to-this.md

@@ -15,5 +15,5 @@ Simply copy the file to the `new` subfolder in your mailer's maildir folder.
 You can use the command-line tool `curl` (provided that your version has IMAP support):
 
 ```sh
-curl -g --user "<email>:<password>" --url "imaps://imap.gmail.com/INBOX" -T /path/to/raw.txt
+curl -g --user "<email>" --url "imaps://imap.gmail.com/INBOX" -T /path/to/raw.txt
 ```