branch: fugitive renamed its functions

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

autoload/airline/extensions/fugitiveline.vim

@@ -3,7 +3,7 @@
 
 scriptencoding utf-8
 
-if !exists('*fugitive#head')
+if !airline#util#has_fugitive()
   finish
 endif