vim-dirvish: Fixed bugs and added better include guards

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.

CHANGELOG.md

@@ -9,6 +9,7 @@ This is the Changelog for the vim-airline project.
     - [vim-lsp](https://github.com/prabirshrestha/vim-lsp) support
     - [zoomwintab](https://github.com/troydm/zoomwintab.vim) support
     - [Vaffle](https://github.com/cocopon/vaffle.vim) support
+    - [vim-dirvish](https://github.com/justinmk/vim-dirvish) support
 - Improvements
   - git branch can also be displayed using [gina.vim](https://github.com/lambdalisue/gina.vim)
   - coc extensions can also show additional status messages