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

autoload/airline/extensions/branch.vim

@@ -122,13 +122,13 @@ function! s:update_hg_branch()
     let file=expand('%:p')
     if !empty(stl) && get(b:, 'airline_do_mq_check', 1)
       if g:airline#init#vim_async
-        call airline#async#get_mq_async(cmd, file)
+        noa call airline#async#get_mq_async(cmd, file)
       elseif has("nvim")
-        call airline#async#nvim_get_mq_async(cmd, file)
+        noa call airline#async#nvim_get_mq_async(cmd, file)
       else
         " remove \n at the end of the command
         let output=system(cmd)[0:-2]
-        call airline#async#mq_output(output, file)
+        noa call airline#async#mq_output(output, file)
       endif
     endif
     " do not do mq check anymore
@@ -199,10 +199,10 @@ function! s:update_untracked()
       " invalidated again before s:update_untracked is called, then we lose the
       " result of the previous call, i.e. the head string is not updated. It
       " doesn't happen often in practice, so we let it be.
-      call airline#async#vim_vcs_untracked(config, file)
+      noa call airline#async#vim_vcs_untracked(config, file)
     else
       " nvim async or vim without job-feature
-      call airline#async#nvim_vcs_untracked(config, file, vcs)
+      noa call airline#async#nvim_vcs_untracked(config, file, vcs)
     endif
   endfor
 endfunction
@@ -242,7 +242,7 @@ function! airline#extensions#branch#head()
 
   if empty(heads)
     if airline#util#has_vcscommand()
-      call VCSCommandEnableBufferSetup()
+      noa call VCSCommandEnableBufferSetup()
       if exists('b:VCSCommandBufferInfo')
         let b:airline_head = s:format_name(get(b:VCSCommandBufferInfo, 0, ''))
       endif