folly/small_vector.h: avoid -Wsign-compare error
folly/small_vector.h: avoid -Wsign-compare error

Summary:
* folly/small_vector.h (moveToUninitialized): Do not mix
signed and unsigned for-loop index/limit.  Instead, eliminate
the intermediate "count" altogether and iterate as long as
"first" is not equal to "last". Otherwise, gcc-4.9 fails with e.g.,
folly/small_vector.h:115:18: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: soren@fb.com

Subscribers: trunkagent, folly-diffs@

FB internal diff: D1767844

Tasks: 5941250

Signature: t1:1767844:1420601608:987fd7f7d44197ed9919910c9b559b37fbe421b6

project-zerus/folly,SammyK/folly,kernelim/folly,brunomorishita/folly,KSreeHarsha/folly,cole14/folly,tomhughes/folly,cole14/folly,bowlofstew/folly,hongliangzhao/folly,floxard/folly,gaoyingie/folly,lifei/folly,romange/folly,doctaweeks/folly,colemancda/folly,bobegir/folly,upsoft/folly,floxard/folly,Eagle-X/folly,bsampath/folly,gavioto/folly,guker/folly,chjp2046/folly,PoisonBOx/folly,project-zerus/folly,zhiweicai/folly,loversInJapan/folly,facebook/folly,guker/folly,juniway/folly,brunomorishita/folly,wildinto/folly,zhiweicai/folly,gavioto/folly,Eagle-X/folly,gaoyingie/folly,zhiweicai/folly,wildinto/folly,CJstar/folly,CJstar/folly,charsyam/folly,upsoft/folly,sonnyhu/folly,reddit/folly,xzmagic/folly,lifei/folly,sonnyhu/folly,hongliangzhao/folly,arg0/folly,clearlylin/folly,raphaelamorim/folly,guker/folly,renyinew/folly,kernelim/folly,romange/folly,wildinto/folly,rklabs/folly,Hincoin/folly,loversInJapan/folly,cppfool/folly,clearlylin/folly,bowlofstew/folly,reddit/folly,stonegithubs/folly,shaobz/folly,clearlylin/folly,yangjin-unique/folly,renyinew/folly,raphaelamorim/folly,constantine001/folly,cole14/folly,theiver9827/folly,leolujuyi/folly,rklabs/folly,stonegithubs/folly,clearlylin/folly,ykarlsbrun/folly,fw1121/folly,PPC64/folly,CJstar/folly,PoisonBOx/folly,leolujuyi/folly,chjp2046/folly,yangjin-unique/folly,loverszhaokai/folly,shaobz/folly,chjp2046/folly,charsyam/folly,Orvid/folly,ykarlsbrun/folly,colemancda/folly,xzmagic/folly,sonnyhu/folly,bsampath/folly,KSreeHarsha/folly,stonegithubs/folly,PPC64/folly,gavioto/folly,tempbottle/folly,bikong2/folly,mqeizi/folly,Eagle-X/folly,stonegithubs/folly,lifei/folly,stonegithubs/folly,arg0/folly,ykarlsbrun/folly,renyinew/folly,loverszhaokai/folly,arg0/folly,tomhughes/folly,Eagle-X/folly,juniway/folly,upsoft/folly,doctaweeks/folly,romange/folly,clearlinux/folly,hongliangzhao/folly,floxard/folly,clearlinux/folly,alexst07/folly,loversInJapan/folly,gaoyingie/folly,brunomorishita/folly,hongliangzhao/folly,SammyK/folly,loverszhaokai/folly,fw1121/folly,loverszhaokai/folly,mqeizi/folly,Orvid/folly,PoisonBOx/folly,clearlinux/folly,romange/folly,lifei/folly,bobegir/folly,sonnyhu/folly,xzmagic/folly,constantine001/folly,kernelim/folly,juniway/folly,Hincoin/folly,PPC64/folly,doctaweeks/folly,raphaelamorim/folly,tempbottle/folly,reddit/folly,KSreeHarsha/folly,upsoft/folly,SeanRBurton/folly,cppfool/folly,Hincoin/folly,leolujuyi/folly,rklabs/folly,doctaweeks/folly,fw1121/folly,SammyK/folly,leolujuyi/folly,bowlofstew/folly,loversInJapan/folly,bikong2/folly,leolujuyi/folly,ykarlsbrun/folly,theiver9827/folly,juniway/folly,doctaweeks/folly,upsoft/folly,facebook/folly,bowlofstew/folly,bikong2/folly,chjp2046/folly,colemancda/folly,clearlylin/folly,guker/folly,sakishum/folly,clearlinux/folly,colemancda/folly,charsyam/folly,gaoyingie/folly,SammyK/folly,nickhen/folly,nickhen/folly,raphaelamorim/folly,floxard/folly,brunomorishita/folly,juniway/folly,bobegir/folly,arg0/folly,mqeizi/folly,raphaelamorim/folly,alexst07/folly,nickhen/folly,romange/folly,reddit/folly,fw1121/folly,rklabs/folly,tempbottle/folly,shaobz/folly,tempbottle/folly,bowlofstew/folly,cppfool/folly,Hincoin/folly,zhiweicai/folly,gavioto/folly,sakishum/folly,theiver9827/folly,arg0/folly,mqeizi/folly,bsampath/folly,xzmagic/folly,guker/folly,alexst07/folly,lifei/folly,Orvid/folly,SeanRBurton/folly,xzmagic/folly,project-zerus/folly,SeanRBurton/folly,bsampath/folly,reddit/folly,bobegir/folly,shaobz/folly,loverszhaokai/folly,Orvid/folly,sonnyhu/folly,constantine001/folly,tempbottle/folly,sakishum/folly,tomhughes/folly,gavioto/folly,CJstar/folly,SammyK/folly,kernelim/folly,theiver9827/folly,facebook/folly,constantine001/folly,Hincoin/folly,floxard/folly,nickhen/folly,charsyam/folly,PoisonBOx/folly,Orvid/folly,facebook/folly,renyinew/folly,SeanRBurton/folly,yangjin-unique/folly,renyinew/folly,PPC64/folly,gaoyingie/folly,ykarlsbrun/folly,bikong2/folly,PPC64/folly,cppfool/folly,tomhughes/folly,bobegir/folly,loversInJapan/folly,cole14/folly,wildinto/folly,KSreeHarsha/folly,hongliangzhao/folly,kernelim/folly,nickhen/folly,colemancda/folly,sakishum/folly,bikong2/folly,SeanRBurton/folly,alexst07/folly,facebook/folly,bsampath/folly,yangjin-unique/folly,sakishum/folly,fw1121/folly,cppfool/folly,theiver9827/folly,shaobz/folly,wildinto/folly,zhiweicai/folly,Eagle-X/folly,tomhughes/folly,chjp2046/folly,rklabs/folly,clearlinux/folly,yangjin-unique/folly,project-zerus/folly,project-zerus/folly,alexst07/folly,PoisonBOx/folly,KSreeHarsha/folly,mqeizi/folly,brunomorishita/folly,constantine001/folly,cole14/folly,CJstar/folly