pgsql: Be a bit more rigorous about how we cache strcoll and strxfrm re

Be a bit more rigorous about how we cache strcoll and strxfrm results.

Commit 0e57b4d8bd9674adaf5747421b3255b85e385534 contained some clever
logic that attempted to make sure that we couldn't get confused about
whether the last thing we cached was a strcoll() result or a strxfrm()
result, but it wasn't quite clever enough, because we can perform
further abbreviations after having already performed some comparisons.
Introduce an explicit flag in the hopes of making this watertight.

Peter Geoghegan, reviewed by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5be94a9eb15a03d8dbc4a5b52760c8776eb3525e

Modified Files
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src/backend/utils/adt/varlena.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)