branch: better display of branch for both hg and git
branch: better display of branch for both hg and git

If a file is edited inside a git repository, which lies within a git
repository, the branch extensions shows 'gitmaster | hgdefault'

To make it more obvious, that we are looking into both repositories
here, use 'git:master | hg:default'

closes #1482

tschundler/dotfiles,rchicoli/vim-airline,unphased/vim-airline,bling/vim-airline,tschundler/dotfiles,tschundler/dotfiles,yzhou61/vim-airline,chrisbra/vim-airline