Implement -F/--fast-index flag
For maildir and MH folders, this flag avoids doing a stat() on each message. This should give a noticeable improvement in indexing speed when the majority of messages are unchanged and indexing is occurring very frequently. However, there is a risk that mairix will not notice changes to an existing message. However, such an omission can easily be cured by a later indexing run without this new flag.
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