pipe: reorder patches based on email subjects

The Message-Id header cannot be relied upon as users can tweak it before
sending. The subject seems a more reliable basis to reorder patches.

Change the logic of sorting. Previously, all messages were required to
look like patches to be sorted. Now if at least one message looks like
a patch, all messages will be sorted by Subject before piping them.
Since `git am` ignores non-patch messages, it should allow piping series
of emails including reviews and comments without getting confusing
errors.

I have tested that this works on multiple series that appeared out of
order in my INBOX with the following command (after marking the
messages):

  :pipe -m sed -n 's/^Subject: //p'

Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>

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