[PATCH]es: algorithm: Avoid moving initial subsequences in std::inplace_merge()

[Duncan P. N. Exon Smith]

r227808 (and r227811) changed `std::inplace_merge()` to meet the
complexity guidelines of the standard, which strictly restricts the
number of calls to the predicate.  Unfortunately, this removed an
optimization that avoided moving any elements if the sequence was
already fully sorted.  Moroever, any initial/final subsequence that was
in the right spot would previously never be touched.

r243530, whose main purpose was to fix a latent self-move bug made more
common by r227808/r227811, already brought half of this optimization
back.  It avoids unnecessarily moving the final subsequence.

These patches close the loop, bringing back the other half of the
optimization without re-introducing the extra predicate call that
r227808 avoided.

0001 is an NFC refactor, while 0002 has the actual change.

(The change in r227811 was to *always* malloc a temporary buffer.  I
wonder if something like this could safely optimize that away as well?)

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