Save branch head to a buffer variable

Save branch head to a buffer-local variable to prevent looking up the VCS head for every status line refresh.
Add CursorHold autocmd to unset airline_head
Merge pull request #469 from russellhancox/patch-1

Save branch head to a buffer variable
remove usage of fugitive#detect as per request

https://github.com/tpope/vim-fugitive/issues/463
restore original directory change detection. also resolves #474.
Now trailing tabs are also reported, fixed bug when trailings with mixed tabs and spaces were being reported as mixed indents, case when count of spaces at the end of tabbed indentation less then tabstop does not reported as mixed indent anymore.
Merge pull request #478 from davits/mixed_indent

Improvements in whitespace plugin