fetch-pack: sort incoming heads list earlier

fetch-pack: sort incoming heads list earlier

Commit 4435968 started sorting heads fed to fetch-pack so
that later commits could use more optimized algorithms;
commit 7db8d53 switched the remove_duplicates function to
such an algorithm.

Of course, the sorting is more effective if you do it
_before_ the algorithm in question.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>