pgsql: Fix lquery's behavior for consecutive '*' items.

Fix lquery's behavior for consecutive '*' items.

Something like "*{2}.*{3}" should presumably mean the same as
"*{5}", but it didn't. Improve that.

Get rid of an undocumented and remarkably ugly (though not, as far as
I can tell, actually unsafe) static variable in favor of passing more
arguments to checkCond().

Reverse-engineer some commentary. This function, like all of ltree,
is still far short of what I would consider the minimum acceptable
level of internal documentation, but at least now it has more than
zero comments.

Although this certainly seems like a bug fix, people might not
thank us for changing query behavior in stable branches, so
no back-patch.

Nikita Glukhov, with cosmetic improvements by me

Discussion: https://postgr.es/m/CAP_rww=waX2Oo6q+MbMSiZ9ktdj6eaJj0cQzNu=Ry2cCDij5fw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9950c8aadf0edd31baec74a729d47d94af636c06

Modified Files
--------------
contrib/ltree/expected/ltree.out | 24 ++++++++
contrib/ltree/lquery_op.c | 121 +++++++++++++++++++++++++++------------
contrib/ltree/ltree.h | 19 +++---
contrib/ltree/sql/ltree.sql | 5 +-
4 files changed, 123 insertions(+), 46 deletions(-)