zsh-completion: remove use of 'head' with negative offset
zsh-completion: remove use of 'head' with negative offset

Some systems - like OSX - don't come with a version of head that
supports a negative value for the -n flag. Such systems get a message
such as this when tab-completing ninja's -d flag:

    ninja -dhead: illegal line count -- -1

Using sed instead should be more universally supported.

dendy/ninja,iwadon/ninja,martine/ninja,jimon/ninja,sxlin/dist_ninja,mgaunard/ninja,mgaunard/ninja,sxlin/dist_ninja,bradking/ninja,moroten/ninja,dendy/ninja,Maratyszcza/ninja-pypi,automeka/ninja,nafest/ninja,autopulated/ninja,bradking/ninja,martine/ninja,Maratyszcza/ninja-pypi,Ju2ender/ninja,fuchsia-mirror/third_party-ninja,ThiagoGarciaAlves/ninja,maruel/ninja,autopulated/ninja,dendy/ninja,tfarina/ninja,ThiagoGarciaAlves/ninja,mydongistiny/ninja,ThiagoGarciaAlves/ninja,ctiller/ninja,ndsol/subninja,moroten/ninja,kissthink/ninja,juntalis/ninja,jimon/ninja,tfarina/ninja,bradking/ninja,kissthink/ninja,juntalis/ninja,martine/ninja,mohamed/ninja,sxlin/dist_ninja,Ju2ender/ninja,kissthink/ninja,bmeurer/ninja,ndsol/subninja,lizh06/ninja,mydongistiny/ninja,nicolasdespres/ninja,synaptek/ninja,ninja-build/ninja,maruel/ninja,atetubou/ninja,mgaunard/ninja,metti/ninja,nicolasdespres/ninja,Qix-/ninja,sxlin/dist_ninja,dorgonman/ninja,lizh06/ninja,sgraham/ninja,Maratyszcza/ninja-pypi,lizh06/ninja,iwadon/ninja,ilor/ninja,tfarina/ninja,hnney/ninja,AoD314/ninja,bradking/ninja,Qix-/ninja,mydongistiny/ninja,mohamed/ninja,juntalis/ninja,automeka/ninja,nico/ninja,ninja-build/ninja,bmeurer/ninja,colincross/ninja,jendrikillner/ninja,AoD314/ninja,hnney/ninja,colincross/ninja,ninja-build/ninja,vvvrrooomm/ninja,moroten/ninja,ctiller/ninja,sgraham/ninja,sxlin/dist_ninja,ThiagoGarciaAlves/ninja,liukd/ninja,vvvrrooomm/ninja,ilor/ninja,jimon/ninja,atetubou/ninja,nafest/ninja,liukd/ninja,iwadon/ninja,automeka/ninja,ignatenkobrain/ninja,dorgonman/ninja,fuchsia-mirror/third_party-ninja,nafest/ninja,mohamed/ninja,ignatenkobrain/ninja,ctiller/ninja,hnney/ninja,ndsol/subninja,ctiller/ninja,iwadon/ninja,vvvrrooomm/ninja,liukd/ninja,bmeurer/ninja,atetubou/ninja,Maratyszcza/ninja-pypi,ignatenkobrain/ninja,bmeurer/ninja,metti/ninja,ignatenkobrain/ninja,synaptek/ninja,lizh06/ninja,sgraham/ninja,colincross/ninja,nico/ninja,kissthink/ninja,hnney/ninja,mgaunard/ninja,mydongistiny/ninja,automeka/ninja,liukd/ninja,nicolasdespres/ninja,nico/ninja,Ju2ender/ninja,colincross/ninja,fuchsia-mirror/third_party-ninja,vvvrrooomm/ninja,dendy/ninja,ndsol/subninja,nicolasdespres/ninja,ilor/ninja,mohamed/ninja,maruel/ninja,jendrikillner/ninja,metti/ninja,martine/ninja,Qix-/ninja,sgraham/ninja,jendrikillner/ninja,ninja-build/ninja,AoD314/ninja,Qix-/ninja,autopulated/ninja,nico/ninja,dorgonman/ninja,metti/ninja,tfarina/ninja,juntalis/ninja,synaptek/ninja,fuchsia-mirror/third_party-ninja,ilor/ninja,nafest/ninja,jendrikillner/ninja,sxlin/dist_ninja,atetubou/ninja,synaptek/ninja,sxlin/dist_ninja,AoD314/ninja,jimon/ninja,dorgonman/ninja,Ju2ender/ninja,maruel/ninja,autopulated/ninja,moroten/ninja