Don't wipe out b:airline_head if fugitive finds the head
Don't wipe out b:airline_head if fugitive finds the head

- Fugitive can edit virtual files that represent objects in git; while
  editing these files, it still reports on the HEAD of the relevant
  repo, which should be displayed as the branch (rather than having the
  branch section disappear)

zekzekus/vim-airline,astronaut1712/vim-airline,kazuoteramoto/vim-airline,maritza05/Vim-Configuration-Files,Archman/vim-airline,tschundler/dotfiles,rchicoli/vim-airline,vim-IDE/vim-airline,elisescu/vim-airline,kimw/vim-airline,RsrchBoy/vim-airline,brownbathrobe/vim-airline,mp42/vim_settings,kalmuthu/vim-airline,HarmtH/vim-airline,Nesta/vim-airline,llinfeng/vim-airline,tafryn/vim-airline,blueyed/vim-airline,baseboxorg/vim-airline,sappo/vim-airline,mildred/vim-airline,tschundler/dotfiles,ozankasikci/vim-airline,glennimoss/vim-airline,sarvex/vim-airline,cchamberlain/vim-airline,cyrixhero/vim-airline,tschundler/dotfiles,mlins/vim-airline,bling/vim-airline,sunyinhui/vim-airline,yubo/vim-airline,zthero/vim-airline,apriendeau/vim-airline,dlanileonardo/vim-airline,jordwalke/vim-airline,lukw00/vim-airline,xxxhycl2010/vim-airline,unphased/vim-airline,NLKNguyen/vim-airline,otaviof/vim-airline,dsouzarc/vim-airline,jmartiuk5/vim-airline,junix/vim-airline,dalguji/vim-airline,gergap/vim-airline,chrisbra/vim-airline,torteropaid/vim-airline,azide0x37/vim-airline,pacoqueen/vim-airline,ornicar/vim-airline,yzhou61/vim-airline,wilywampa/vim-airline,PatrickCoffey/vim-airline,ForrestAlfred/vim-airline,acasajus/vim-airline,ghickman/vim-airline,ahaasler/vim-airline