fugitive: remove old fugitive test

As mentioned by @tpope, remove the old test for the autoloaded function
fugitivie#head() and instead use consistently FugitiveHead() everywhere

[delete] %bd command

[add] init.vimspec

[update] init.vimspec

[add] parts.vim

[add] section.vimspec

[add] themes.vimspec

[add] util.vimspec

[delete] vim-vspec

test/extensions_tabline.vimspec

@@ -1,7 +1,4 @@
 Describe extensions_tabline.vim
-  Before all
-    :%bd
-  End
 
   It should use a tabline
     e! file1