Fix the bug in MergeUtil which causes mixing values of different keys.
Fix the bug in MergeUtil which causes mixing values of different keys.

Summary: Fix the bug in MergeUtil which causes mixing values of different keys.

Test Plan:
stringappend_test
make all check

Reviewers: haobo, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17235

ylong/rocksdb,wlqGit/rocksdb,JackLian/rocksdb,Applied-Duality/rocksdb,vashstorm/rocksdb,JoeWoo/rocksdb,ylong/rocksdb,NickCis/rocksdb,makelivedotnet/rocksdb,ryneli/rocksdb,bbiao/rocksdb,fengshao0907/rocksdb,wenduo/rocksdb,flabby/rocksdb,norton/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,tsheasha/rocksdb,lgscofield/rocksdb,temicai/rocksdb,Vaisman/rocksdb,makelivedotnet/rocksdb,vmx/rocksdb,temicai/rocksdb,Applied-Duality/rocksdb,alihalabyah/rocksdb,RyanTech/rocksdb,JoeWoo/rocksdb,facebook/rocksdb,facebook/rocksdb,Vaisman/rocksdb,OverlordQ/rocksdb,JackLian/rocksdb,zhangpng/rocksdb,alihalabyah/rocksdb,dkorolev/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,temicai/rocksdb,hobinyoon/rocksdb,wlqGit/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,skunkwerks/rocksdb,Andymic/rocksdb,caijieming-baidu/rocksdb,flabby/rocksdb,vashstorm/rocksdb,alihalabyah/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,sorphi/rocksdb,biddyweb/rocksdb,vmx/rocksdb,Andymic/rocksdb,biddyweb/rocksdb,dkorolev/rocksdb,luckywhu/rocksdb,bbiao/rocksdb,siddhartharay007/rocksdb,norton/rocksdb,jalexanderqed/rocksdb,Applied-Duality/rocksdb,vashstorm/rocksdb,bbiao/rocksdb,dkorolev/rocksdb,zhangpng/rocksdb,facebook/rocksdb,skunkwerks/rocksdb,rDSN-Projects/rocksdb.replicated,tizzybec/rocksdb,JoeWoo/rocksdb,jalexanderqed/rocksdb,virtdb/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,flabby/rocksdb,mbarbon/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,vashstorm/rocksdb,siddhartharay007/rocksdb,NickCis/rocksdb,OverlordQ/rocksdb,jalexanderqed/rocksdb,wlqGit/rocksdb,tsheasha/rocksdb,anagav/rocksdb,norton/rocksdb,JoeWoo/rocksdb,norton/rocksdb,lgscofield/rocksdb,tschottdorf/rocksdb,JohnPJenkins/rocksdb,NickCis/rocksdb,tsheasha/rocksdb,fengshao0907/rocksdb,temicai/rocksdb,SunguckLee/RocksDB,kaschaeffer/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,amyvmiwei/rocksdb,IMCG/RcoksDB,SunguckLee/RocksDB,IMCG/RcoksDB,kaschaeffer/rocksdb,vmx/rocksdb,wenduo/rocksdb,JohnPJenkins/rocksdb,tizzybec/rocksdb,wenduo/rocksdb,biddyweb/rocksdb,dkorolev/rocksdb,wat-ze-hex/rocksdb,anagav/rocksdb,Vaisman/rocksdb,Andymic/rocksdb,bbiao/rocksdb,mbarbon/rocksdb,tizzybec/rocksdb,luckywhu/rocksdb,vashstorm/rocksdb,wskplho/rocksdb,Andymic/rocksdb,IMCG/RcoksDB,skunkwerks/rocksdb,bbiao/rocksdb,ylong/rocksdb,biddyweb/rocksdb,SunguckLee/RocksDB,zhangpng/rocksdb,virtdb/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,alihalabyah/rocksdb,RyanTech/rocksdb,vashstorm/rocksdb,JohnPJenkins/rocksdb,JackLian/rocksdb,msb-at-yahoo/rocksdb,Andymic/rocksdb,norton/rocksdb,zhangpng/rocksdb,RyanTech/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,luckywhu/rocksdb,geraldoandradee/rocksdb,wat-ze-hex/rocksdb,geraldoandradee/rocksdb,ryneli/rocksdb,luckywhu/rocksdb,Applied-Duality/rocksdb,fengshao0907/rocksdb,makelivedotnet/rocksdb,JohnPJenkins/rocksdb,wat-ze-hex/rocksdb,amyvmiwei/rocksdb,SunguckLee/RocksDB,wskplho/rocksdb,wat-ze-hex/rocksdb,skunkwerks/rocksdb,anagav/rocksdb,tschottdorf/rocksdb,alihalabyah/rocksdb,siddhartharay007/rocksdb,NickCis/rocksdb,wskplho/rocksdb,IMCG/RcoksDB,wlqGit/rocksdb,caijieming-baidu/rocksdb,wlqGit/rocksdb,rDSN-Projects/rocksdb.replicated,ryneli/rocksdb,wenduo/rocksdb,dkorolev/rocksdb,facebook/rocksdb,flabby/rocksdb,jalexanderqed/rocksdb,ylong/rocksdb,geraldoandradee/rocksdb,msb-at-yahoo/rocksdb,Vaisman/rocksdb,temicai/rocksdb,temicai/rocksdb,Applied-Duality/rocksdb,siddhartharay007/rocksdb,facebook/rocksdb,amyvmiwei/rocksdb,ylong/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,JoeWoo/rocksdb,lgscofield/rocksdb,lgscofield/rocksdb,OverlordQ/rocksdb,amyvmiwei/rocksdb,siddhartharay007/rocksdb,bbiao/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,makelivedotnet/rocksdb,wskplho/rocksdb,makelivedotnet/rocksdb,ryneli/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,siddhartharay007/rocksdb,wat-ze-hex/rocksdb,geraldoandradee/rocksdb,RyanTech/rocksdb,dkorolev/rocksdb,tsheasha/rocksdb,virtdb/rocksdb,zhangpng/rocksdb,zhangpng/rocksdb,norton/rocksdb,facebook/rocksdb,temicai/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,tizzybec/rocksdb,fengshao0907/rocksdb,hobinyoon/rocksdb,RyanTech/rocksdb,sorphi/rocksdb,sorphi/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,tizzybec/rocksdb,fengshao0907/rocksdb,IMCG/RcoksDB,anagav/rocksdb,caijieming-baidu/rocksdb,JackLian/rocksdb,JohnPJenkins/rocksdb,msb-at-yahoo/rocksdb,RyanTech/rocksdb,tizzybec/rocksdb,kaschaeffer/rocksdb,wskplho/rocksdb,norton/rocksdb,ylong/rocksdb,hobinyoon/rocksdb,JoeWoo/rocksdb,mbarbon/rocksdb,hobinyoon/rocksdb,luckywhu/rocksdb,luckywhu/rocksdb,JackLian/rocksdb,hobinyoon/rocksdb,amyvmiwei/rocksdb,caijieming-baidu/rocksdb,lgscofield/rocksdb,IMCG/RcoksDB,msb-at-yahoo/rocksdb,wlqGit/rocksdb,Vaisman/rocksdb,virtdb/rocksdb,bbiao/rocksdb,Applied-Duality/rocksdb,wenduo/rocksdb,rDSN-Projects/rocksdb.replicated,zhangpng/rocksdb,Andymic/rocksdb,JackLian/rocksdb,virtdb/rocksdb,geraldoandradee/rocksdb,lgscofield/rocksdb,amyvmiwei/rocksdb,jalexanderqed/rocksdb,tschottdorf/rocksdb,tschottdorf/rocksdb,sorphi/rocksdb,tschottdorf/rocksdb,anagav/rocksdb,JoeWoo/rocksdb,wskplho/rocksdb,alihalabyah/rocksdb,SunguckLee/RocksDB,flabby/rocksdb,rDSN-Projects/rocksdb.replicated,Applied-Duality/rocksdb,kaschaeffer/rocksdb,rDSN-Projects/rocksdb.replicated,luckywhu/rocksdb,skunkwerks/rocksdb,biddyweb/rocksdb,wenduo/rocksdb,wskplho/rocksdb,caijieming-baidu/rocksdb,mbarbon/rocksdb,tizzybec/rocksdb,dkorolev/rocksdb,amyvmiwei/rocksdb,ylong/rocksdb,facebook/rocksdb,biddyweb/rocksdb,bbiao/rocksdb,anagav/rocksdb,sorphi/rocksdb,virtdb/rocksdb,flabby/rocksdb,msb-at-yahoo/rocksdb,sorphi/rocksdb,SunguckLee/RocksDB,geraldoandradee/rocksdb,facebook/rocksdb,virtdb/rocksdb,kaschaeffer/rocksdb,NickCis/rocksdb,wlqGit/rocksdb,geraldoandradee/rocksdb,tsheasha/rocksdb,makelivedotnet/rocksdb,NickCis/rocksdb,JohnPJenkins/rocksdb,mbarbon/rocksdb,IMCG/RcoksDB,ryneli/rocksdb,alihalabyah/rocksdb,Vaisman/rocksdb,JohnPJenkins/rocksdb,lgscofield/rocksdb,norton/rocksdb,caijieming-baidu/rocksdb,biddyweb/rocksdb,tschottdorf/rocksdb,skunkwerks/rocksdb,JackLian/rocksdb,rDSN-Projects/rocksdb.replicated,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,ryneli/rocksdb,fengshao0907/rocksdb,rDSN-Projects/rocksdb.replicated,sorphi/rocksdb,NickCis/rocksdb,makelivedotnet/rocksdb,anagav/rocksdb,kaschaeffer/rocksdb,tsheasha/rocksdb,mbarbon/rocksdb,OverlordQ/rocksdb,fengshao0907/rocksdb,RyanTech/rocksdb,siddhartharay007/rocksdb,ryneli/rocksdb,vashstorm/rocksdb,msb-at-yahoo/rocksdb