When doing a DISTINCT query using an index, try to use the index to skip ahead

to the next distinct element, rather than doing a full scan of the index.
(This is the "skip-ahead-distinct" optimization.)

FossilOrigin-Name: f489b5bb6b35665befdd411c2c55df5258e83cba265d8c4427ba22529cf882a4

diff --cc manifest
index 6a6c5eab167da2e840479607d0ecb4a829ecc607,91fde0d9988fe6f9024eb9cb4e4fe6a2283f91d3..658ad9ece0743b7496420e62308222f4feefe888
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,8 -1,8 +1,8 @@@
- C Make\sUSE_FULLWARN=1\sthe\sdefault\sfor\sMSVC\sand\sfix\sharmless\scompiler\swarnings.
- D 2017-04-14T14:50:34.600
 -C Fix\sa\scouple\sof\sunreachable\sbranches.
 -D 2017-04-14T00:45:51.643
++C When\sdoing\sa\sDISTINCT\squery\susing\san\sindex,\stry\sto\suse\sthe\sindex\sto\sskip\sahead\nto\sthe\snext\sdistinct\selement,\srather\sthan\sdoing\sa\sfull\sscan\sof\sthe\sindex.\n(This\sis\sthe\s"skip-ahead-distinct"\soptimization.)
++D 2017-04-14T17:30:43.741
  F Makefile.in 1cc758ce3374a32425e4d130c2fe7b026b20de5b8843243de75f087c0a2661fb
  F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 -F Makefile.msc a4c0613a18663bda56d8cf76079ab6590a7c3602e54befb4bbdef76bcaa38b6a
 +F Makefile.msc 6a8c838220f7c00820e1fc0ac1bccaaa8e5676067e1dbfa1bafa7a4ffecf8ae6
  F README.md 2b15fae33852f2f53996774c21fb41e1d94181c4401a0e43ac93e11f2cc901b9
  F VERSION 0a0e02e16b44ea735b40118fc844311b2ab0d35b25fbeda5120aee62f973f663
  F aclocal.m4 a5c22d164aff7ed549d53a90fa56d56955281f50
@@@ -1571,7 -1572,7 +1572,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P 5159cb8f2bcfb1f7114786ba23082d2b91a26e3a7ddfae75f8bd362792949d5e
- R 3270eeed59983be191492d424a0b906b
- U mistachkin
- Z edc8ddb73a9c8e57e014e4878b2c6186
 -P ce1e2b88777e00a82c04abe5ba35eec81b5f324e462f099cd00b21054f369688
 -R 0ca9ecc0c2f53a31b4c154be4d5df4e8
++P 6bf673767b8e5cedef1acff795cbe524fab8db2525c06424db4e038934a33936 1aa0ea8db7580aff8bed2f78117ed50019e5545cef48cab782f512a2599fd0c5
++R 23e98d199cf27acff0cc1d4208755e74
++T +closed 1aa0ea8db7580aff8bed2f78117ed50019e5545cef48cab782f512a2599fd0c5
+ U drh
 -Z ac554ac9c4b24b721f6e08016943d6a5
++Z 6edb0fabfc88ce2a863c335cf5598131

diff --cc manifest.uuid
index 39cfc39f282c87ae99884a259b08840629d3dcbd,bf75741857ed5f854c8999ec7acb2acd9daad3b8..dedaf39dc7c3aa9992f00af130dbc9465446725f
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 6bf673767b8e5cedef1acff795cbe524fab8db2525c06424db4e038934a33936
 -1aa0ea8db7580aff8bed2f78117ed50019e5545cef48cab782f512a2599fd0c5
++f489b5bb6b35665befdd411c2c55df5258e83cba265d8c4427ba22529cf882a4