Several extensions trigger custom actions. Make sure, those actions are
only performed, if airline is actually active and not temporarily
disabled using e.g. :AirlineToggle
The 'autochdir' option could be toggled on later in a session.
So do not set the has_autochdir flag once the file is read, but evaluate
it every time the corresponding function is called.
While at it, get rid of the s:fmod variable. Instead let a helper
function return the correct modifier flags depending on the value of the
'autochdir' option.
this caused that the branch extension was not correctly working anymore
because of a refactoring of fugitive tpope/vim-fugitive@5d11ff7
Solution: Move the existence check for the fugitive plugin into a
separate function and call it from there in all places that check the
fugitive plugin. Do the same for lawrencium and vcscommand check.
fixes#605#1739
