mailsplit: sort maildir filenames more cleverly

A maildir does not technically record the order in which
items were placed into it. That means that when applying a
patch series from a maildir, we may get the patches in the
wrong order. We try to work around this by sorting the
filenames. Unfortunately, this may or may not work depending
on the naming scheme used by the writer of the maildir.

For instance, mutt will write:

  ${epoch_seconds}.${pid}_${seq}.${host}

where we have:

  - epoch_seconds: timestamp at which entry was written
  - pid: PID of writing process
  - seq: a sequence number to ensure uniqueness of filenames
  - host: hostname

None of the numbers are zero-padded. Therefore, when we sort
the names as byte strings, entries that cross a digit
boundary (e.g., 10) will sort out of order.  In the case of
timestamps, it almost never matters (because we do not cross
a digit boundary in the epoch time very often these days).
But for the sequence number, a 10-patch series would be
ordered as 1, 10, 2, 3, etc.

To fix this, we can use a custom sort comparison function
which traverses each string, comparing chunks of digits
numerically, and otherwise doing a byte-for-byte comparison.
That would sort:

  123.456_1.bar
  123.456_2.bar
  ...
  123.456_10.bar

according to the sequence number. Since maildir does not
define a filename format, this is really just a heuristic.
But it happens to work for mutt, and there is a reasonable
chance that it will work for other writers, too (at least as
well as a straight sort).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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