feat: Use prefix consistently for all commands

Downside: Git must be installed in the container that is invoked with
the prefix.
Advantage: Everything is performed with the same system. E.g. no
problems with different versions of Bob inside and outside of the
container.

plugin/vim-bob.vim

@@ -48,7 +48,11 @@ function! s:EscapeForMakeprg(text)
 endfunction
 
 function! s:Init(path)
-	let l:bob_base_path = empty(a:path) ? getcwd() : fnamemodify(a:path, ':p:h')
+	let l:bob_base_path = !empty(a:path) ?
+				\ fnamemodify(a:path, ':p:h') :
+				\ empty(g:bob_prefix) ?
+				\     getcwd() : trim(system(g:bob_prefix . " pwd"))
+	echomsg l:bob_base_path
 	" not using `--directory` but `cd` instead, because when running inside of
 	" a container via `g:bob_prefix` we would pass the path on the host to Bob
 	" running in the container, where the path is very likely different