branch: use :noa before calling async func
branch: use :noa before calling async func

for some reasons, calling async functions might cause an error, if a
SourcePre command has been defined. So explicitly call the functions
using `:noa` modifier to prevent triggering the SourcePre autocommand.

fixes #1742

yzhou61/vim-airline,chrisbra/vim-airline,bling/vim-airline,rchicoli/vim-airline