Maildir: Use hardlink for copies of a message
If I have multiple filters that copy a message to different local-account folders, those copies should be hard links to the same actual file. If the copy is from a local-account folder, indeed, those copies should be hard links of the original message.
So for example, copy from local-account INBOX to INBOX->foo and INBOX-bar (i.e. even if in two different filters) should result in a single file on the disk with 3 hard links.