On an UPDATE, try to overwrite an existing btree cell with the modified

content, if the old and new cell are the same size.  Use memcmp() first
to avoid dirtying pages that are unchanged.

FossilOrigin-Name: 5887d8beb502ad62689d31b850f46ab50831a1e9db36adf20d55ad45619d207e

diff --cc manifest
index 5ce3d3060f9a5281da64b71a4ed4b2ed8751ea01,363640929f2d43582f2cc8266df8bcfb9a2e428a..6862957f3f78cefb04fb02f34873e852511c2dc4
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Fix\sharmless\scompiler\swarnings\sassociated\swith\sthe\snew\sEXPLAIN\sQUERY\sPLAN\slogic.
- D 2018-05-07T11:37:34.277
 -C Fix\sharmless\scompiler\swarnings\sin\sthe\scell-overwrite\slogic.
 -D 2018-05-07T11:29:59.385
++C On\san\sUPDATE,\stry\sto\soverwrite\san\sexisting\sbtree\scell\swith\sthe\smodified\ncontent,\sif\sthe\sold\sand\snew\scell\sare\sthe\ssame\ssize.\s\sUse\smemcmp()\sfirst\nto\savoid\sdirtying\spages\sthat\sare\sunchanged.
++D 2018-05-07T11:48:22.641
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 5ce9343cba9c189046f1afe6d2bcc1f68079439febc05267b98aec6ecc752439
@@@ -434,7 -434,7 +434,7 @@@ F src/auth.c 6277d63837357549fe14e72349
  F src/backup.c faf17e60b43233c214aae6a8179d24503a61e83b
  F src/bitvec.c 17ea48eff8ba979f1f5b04cc484c7bb2be632f33
  F src/btmutex.c 8acc2f464ee76324bf13310df5692a262b801808984c1b79defb2503bbafadb6
- F src/btree.c 79982b0779c27e5ccde8c3e540f14b4e82586d6ec0394a7eb08be72d20c3e894
 -F src/btree.c b4ca692c0c557bba653412a0c3347f0e5c140e759f0d8349383b7de7281ad99e
++F src/btree.c 6be0267a7da8ca04c466094ca9d9a8a26333e175cff794afe5138aeec0cdb7c2
  F src/btree.h 0866c0a08255142ea0e754aabd211c843cab32045c978a592a43152405ed0c84
  F src/btreeInt.h 620ab4c7235f43572cf3ac2ac8723cbdf68073be4d29da24897c7b77dda5fd96
  F src/build.c 0c2be5839f22aa2938f217c6c6c2120d9fc96872a546a37541a8271541cb355e
@@@ -1727,7 -1727,7 +1727,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P b348d1193a7a3ed4d3e656b6cc95a41f87eae29222e0723850a5eb5ffabffd00
- R d09bdc73f56d4d80e2806c4c66a4e25a
 -P 9650f71b82ca1b3759e776bbd8a2e8ca61c51f80038e37c9c9a675a8c13b0b06
 -R 0f12e4904e55641cfcfe9534f6ff08d6
++P 374d8e264487b0437a8d995ced1bc026a92d495a2d0568f65f033e9ebe11d0e2 3e11dc3183bc3e8ec49af244a8e8b3e07d12f7a2e59028b2bf64ce0ab589a91f
++R 891ce8c2c78f1f487f80460b8c248fd0
++T +closed 3e11dc3183bc3e8ec49af244a8e8b3e07d12f7a2e59028b2bf64ce0ab589a91f
  U drh
- Z 7e3b1491b97d0e22d20f39d7466da090
 -Z eb245c54fb26ccb37327155d2c7d476e
++Z 7955ef5d032aea210d99e8bc24e906bc

diff --cc manifest.uuid
index a55df92edefc4f785a317c5931d12ccd32ebfb17,adb6239ec15b0cec397b149a44a90bf55cf754f0..3fa7a09c8ab979bff8e5176bed5b7c96681c2ca9
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 374d8e264487b0437a8d995ced1bc026a92d495a2d0568f65f033e9ebe11d0e2
 -3e11dc3183bc3e8ec49af244a8e8b3e07d12f7a2e59028b2bf64ce0ab589a91f
++5887d8beb502ad62689d31b850f46ab50831a1e9db36adf20d55ad45619d207e

diff --cc src/btree.c
Simple merge