Add APIs to the sessions module for "rebasing" changesets.

FossilOrigin-Name: 509506c76b7c104961826721013889d6c6b2ed9b563dcd029e0cb5cb5c34693a

diff --cc manifest
index 3da7a1096c5c7126ad3e2fa89467f64f65613d8e,82229d2d93113b0cfb818d175ba827aa9c9e8bb7..c5d48b83be7419565be60e4903603dc64c700c8f
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Fix\san\sRBU\sproblem\scausing\serrors\swhen\supdating\stables\swith\sdefault\scollation\nsequences\sthat\srequire\squoting\s(e.g.\sCOLLATE\s"ICU_root-u-kn-on").
- D 2018-03-22T17:13:44.592
 -C Remove\ssome\sunused\scode\sfrom\ssqlite3rebaser_rebase().
 -D 2018-03-22T20:11:14.074
++C Add\sAPIs\sto\sthe\ssessions\smodule\sfor\s"rebasing"\schangesets.
++D 2018-03-22T20:35:20.917
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 7016fc56c6b9bfe5daac4f34be8be38d8c0b5fab79ccbfb764d3b23bf1c6fff3
@@@ -1716,7 -1716,7 +1717,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P 901cb3b6a2c8d0cc33bd34ec1dbeea49c779ae1ac3ed6733dd9826c8e0eb80c8
- R ea4beed5a6f5a13ca5aa6b2ab2f884d0
 -P c8e7b5a061a3f2fbd9072530177b80f917b39ab5d7cd6acb0a221ab33e78a5cd
 -R 1ada6392e139233f598c5ef10ceecba8
++P eb4f452e354065d610ff57a6a9312ad119b6b0cc467f9dff105f0718bc27ef01 07cc955eab0e993a75be82d58e17ca53c8abbcaf851983d235049599c19e582f
++R 27f6cf97018270780b80846440bc882c
++T +closed 07cc955eab0e993a75be82d58e17ca53c8abbcaf851983d235049599c19e582f
  U dan
- Z 1a6cc6f516e2cd588ac4c5f3e803854c
 -Z 1eb1ab24b6bf6dc6b9f05aed875e4365
++Z c2acdc4842096b171e69d1edd2d47e40

diff --cc manifest.uuid
index f5e9e9a1dc9c8a4ebabb07f51dc40d1afb338c54,eafe5b75317b1ea33b37b4e9f16ea2b968272e90..10a780dc6db5301382c32e0413f07bc579322940
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- eb4f452e354065d610ff57a6a9312ad119b6b0cc467f9dff105f0718bc27ef01
 -07cc955eab0e993a75be82d58e17ca53c8abbcaf851983d235049599c19e582f
++509506c76b7c104961826721013889d6c6b2ed9b563dcd029e0cb5cb5c34693a