This was causing persistent errors when you run, for example,
```
ls | nvim -c 'setf dirvish'
```
as dirvish will not yet have set that local variable yet airline will
attempt to use it.
Fixes issue #2142.
These changes primarily focused on making the include guards more robust
and simplify how the path for w:airline_section_c was generated.
Credits goes to @kazukazuinaina for improving on my original
implementation.
Matched VCS format of Netrw to only show branch name.
Filetype hidden as it is a duplicate on the left side.
File encoding hidden as it does not matter in dirvish buffer.
Removed unnecessary current column information
