Add support for the sqlite3_vtab_distinct() interface. Virtual table

implementations can use this API to determine more detail about the ordering
requirements needed by the query plan and perhaps reduce the amount of
work required to compute a correct answer.  This is an optimization
opportunity for the virtual table implementation.  The correct answer should
still be obtained (though perhaps more slowly) even if sqlite3_vtab_distinct()
is ignored.

FossilOrigin-Name: e4caf1e3932b1bd0dea072df7fc9458aed98c84ea397b6948b89292603949c41

diff --cc manifest
index 577199e3ba07847465fc7f5dd32dc2b81bb61659,882cfbc89f77ff9cebc9e825ffc5c8fb2b816b07..c9f7856650539293ff23f1fd3a998136ef2942c4
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Update\sevidence\smarks\sfor\sthe\slatest\schanges\sto\sthe\sdocumentation.
- D 2022-01-21T19:26:18.123
 -C Bring\ssqlite3_vtab_distinct()\sup\sto\sspec\sso\sthat\sit\sworks\sas\sdescribed\sin\sthe\ndocumentation.
 -D 2022-01-22T20:45:57.210
++C Add\ssupport\sfor\sthe\ssqlite3_vtab_distinct()\sinterface.\s\sVirtual\stable\nimplementations\scan\suse\sthis\sAPI\sto\sdetermine\smore\sdetail\sabout\sthe\sordering\nrequirements\sneeded\sby\sthe\squery\splan\sand\sperhaps\sreduce\sthe\samount\sof\nwork\srequired\sto\scompute\sa\scorrect\sanswer.\s\sThis\sis\san\soptimization\nopportunity\sfor\sthe\svirtual\stable\simplementation.\s\sThe\scorrect\sanswer\sshould\nstill\sbe\sobtained\s(though\sperhaps\smore\sslowly)\seven\sif\ssqlite3_vtab_distinct()\nis\signored.
++D 2022-01-22T22:28:32.461
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@@ -1940,8 -1941,8 +1941,9 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
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+ R 8c92b45cd50ac9ababc3056da5861966
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  # Remove this line to create a well-formed Fossil manifest.

diff --cc manifest.uuid
index 7a58964939d27cae96dcf9d650a9c84986d6b917,e96d220661d5481e0eabd5488ce498ce1f32f8a5..4cfc6dc89d5180e9f70bd200d22980633a88c9d8
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 19247e919fab9748cae561cb12c4c3c106064390a37e32e724d9a9066cfaff8e
 -4289edf3c5e32a05b51f232020099b33f6f5e79b0ceca2b96baf1186168d9af6
++e4caf1e3932b1bd0dea072df7fc9458aed98c84ea397b6948b89292603949c41