plan9port: add HEAD support
plan9port: add HEAD support

This allows users to do `brew install --HEAD plan9port`. This is needed
because the latest release on https://code.google.com/p/plan9port/downloads/list
has broken GUI integration in OS X 10.11.

Closes #46694.

Signed-off-by: Dominyk Tiller <53e438f55903875d07efdd98a8aaf887e7208dd3@gmail.com>

SuperNEMO-DBD/cadfaelbrew,deorth/homebrew,AGWA-forks/homebrew,scorphus/homebrew,xcezx/homebrew,yangj1e/homebrew,dkotvan/homebrew,gnubila-france/linuxbrew,dalanmiller/homebrew,galaxy001/homebrew,elgertam/homebrew,apjanke/homebrew,AlexejK/homebrew,lvicentesanchez/homebrew,georgschoelly/homebrew,yangj1e/homebrew,yazuuchi/homebrew,kenips/homebrew,3van/homebrew,osimola/homebrew,Gasol/homebrew,megahall/homebrew,hongkongkiwi/homebrew,mactkg/homebrew,petere/homebrew,3van/homebrew,jiashuw/homebrew,cchacin/homebrew,MartinDelille/homebrew,kodabb/homebrew,keith/homebrew,koraktor/homebrew,scorphus/homebrew,totalvoidness/homebrew,BrewTestBot/homebrew,ilovezfs/homebrew,dalanmiller/homebrew,scpeters/homebrew,ebouaziz/linuxbrew,cchacin/homebrew,ryanfb/homebrew,danieroux/homebrew,cscetbon/homebrew,dalanmiller/homebrew,Linuxbrew/linuxbrew,knpwrs/homebrew,benjaminfrank/homebrew,Asuranceturix/homebrew,frodeaa/homebrew,totalvoidness/homebrew,mkrapp/homebrew,apjanke/homebrew,dunn/homebrew,treyharris/homebrew,hakamadare/homebrew,dongcarl/homebrew,apjanke/homebrew,AGWA-forks/homebrew,Homebrew/homebrew,hakamadare/homebrew,MartinDelille/homebrew,zeha/homebrew,knpwrs/homebrew,sometimesfood/homebrew,menivaitsi/homebrew,influxdata/homebrew,chadcatlett/homebrew,ento/homebrew,teslamint/homebrew,base10/homebrew,jab/homebrew,MoSal/homebrew,razamatan/homebrew,miketheman/homebrew,Red54/homebrew,DoomHammer/linuxbrew,petere/homebrew,blogabe/homebrew,benjaminfrank/homebrew,jamer/homebrew,reelsense/homebrew,keith/homebrew,chadcatlett/homebrew,sjackman/linuxbrew,vigo/homebrew,sferik/homebrew,jconley/homebrew,dunn/homebrew,elyscape/homebrew,dickeyxxx/homebrew,gnubila-france/linuxbrew,filcab/homebrew,megahall/homebrew,saketkc/linuxbrew,ryanfb/homebrew,dutchcoders/homebrew,drewwells/homebrew,yumitsu/homebrew,optikfluffel/homebrew,afh/homebrew,yangj1e/homebrew,dickeyxxx/homebrew,LinusU/homebrew,benjaminfrank/homebrew,hanxue/homebrew,LinusU/homebrew,razamatan/homebrew,bukzor/linuxbrew,SuperNEMO-DBD/cadfaelbrew,yumitsu/homebrew,gnubila-france/linuxbrew,dalanmiller/homebrew,tutumcloud/homebrew,youtux/homebrew,elgertam/homebrew,Angeldude/linuxbrew,influxdata/homebrew,vigo/homebrew,gawbul/homebrew,iandennismiller/homebrew,MartinDelille/homebrew,AGWA-forks/homebrew,utzig/homebrew,skinny-framework/homebrew,galaxy001/homebrew,zeha/homebrew,recruit-tech/homebrew,ehamberg/homebrew,royhodgman/homebrew,decors/homebrew,jsjohnst/homebrew,arnested/homebrew,miketheman/homebrew,SteveClement/homebrew,drbenmorgan/linuxbrew,drbenmorgan/linuxbrew,BrewTestBot/homebrew,AlexejK/homebrew,johanhammar/homebrew,mactkg/homebrew,redpen-cc/homebrew,blogabe/homebrew,Angeldude/linuxbrew,johanhammar/homebrew,sometimesfood/homebrew,cosmo0920/homebrew,zeha/homebrew,ExtremeMan/homebrew,redpen-cc/homebrew,scorphus/homebrew,Angeldude/linuxbrew,thinker0/homebrew,SteveClement/homebrew,drewwells/homebrew,dickeyxxx/homebrew,heinzf/homebrew,koraktor/homebrew,dericed/homebrew,youtux/homebrew,heinzf/homebrew,menivaitsi/homebrew,jsjohnst/homebrew,Gasol/homebrew,dickeyxxx/homebrew,haihappen/homebrew,dongcarl/homebrew,osimola/homebrew,davydden/linuxbrew,dgageot/homebrew,yumitsu/homebrew,Linuxbrew/linuxbrew,pdxdan/homebrew,filcab/homebrew,cprecioso/homebrew,brunchboy/homebrew,cprecioso/homebrew,feelpp/homebrew,Chilledheart/homebrew,lrascao/homebrew,elgertam/homebrew,ralic/homebrew,dgageot/homebrew,liamstask/homebrew,chadcatlett/homebrew,mmizutani/homebrew,teslamint/homebrew,frodeaa/homebrew,gicmo/homebrew,muellermartin/homebrew,reelsense/linuxbrew,decors/homebrew,ento/homebrew,joshfriend/homebrew,elyscape/homebrew,sjackman/linuxbrew,zchee/homebrew,mobileoverlord/homebrew-1,mmizutani/homebrew,utzig/homebrew,cscetbon/homebrew,ralic/homebrew,jconley/homebrew,tschoonj/homebrew,jconley/homebrew,michaKFromParis/homebrew-sparks,alebcay/homebrew,codeout/homebrew,emcrisostomo/homebrew,reelsense/homebrew,LinusU/homebrew,cscetbon/homebrew,esamson/homebrew,princeofdarkness76/homebrew,tschoonj/homebrew,kodabb/homebrew,elyscape/homebrew,treyharris/homebrew,jab/homebrew,afh/homebrew,MartinDelille/homebrew,eighthave/homebrew,bukzor/linuxbrew,menivaitsi/homebrew,utzig/homebrew,sferik/homebrew,alebcay/homebrew,pdxdan/homebrew,danieroux/homebrew,3van/homebrew,iostat/homebrew2,influxdb/homebrew,skinny-framework/homebrew,adamliter/homebrew,dkotvan/homebrew,geoff-codes/homebrew,xcezx/homebrew,Red54/homebrew,influxdata/homebrew,hakamadare/homebrew,knpwrs/homebrew,booi/homebrew,blogabe/homebrew,ilovezfs/homebrew,iandennismiller/homebrew,Asuranceturix/homebrew,apjanke/homebrew,Red54/homebrew,MoSal/homebrew,SteveClement/homebrew,Angeldude/linuxbrew,iostat/homebrew2,Homebrew/homebrew,skatsuta/homebrew,skinny-framework/homebrew,base10/homebrew,hanxue/homebrew,jarrettmeyer/homebrew,zhipeng-jia/homebrew,xcezx/homebrew,marcwebbie/homebrew,quantumsteve/homebrew,josa42/homebrew,zchee/homebrew,BrewTestBot/homebrew,Homebrew/linuxbrew,joshua-rutherford/homebrew,BrewTestBot/homebrew,nelstrom/homebrew,pdxdan/homebrew,dutchcoders/homebrew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