pgsql: Don't require sort support functions to provide a comparator.

Don't require sort support functions to provide a comparator.

This could be useful for datatypes like text, where we might want
to optimize for some collations but not others.  However, this patch
doesn't introduce any new sortsupport functions that work this way;
it merely revises the code so that future patches may do so.

Patch by me.  Review by Peter Geoghegan.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1d41739e5a04b0e93304d24d864b6bfa3efc45f2

Modified Files
--------------
src/backend/executor/nodeMergejoin.c |    8 ++---
src/backend/utils/cache/lsyscache.c  |   56 ----------------------------------
src/backend/utils/sort/sortsupport.c |   42 ++++++++++++++++++-------
src/include/utils/lsyscache.h        |    2 --
4 files changed, 35 insertions(+), 73 deletions(-)