Fix #1051 - Support ash and dash for shellcheck and the sh linter

ale_linters/sh/shellcheck.vim

@@ -23,15 +23,20 @@ function! ale_linters#sh#shellcheck#GetDialectArgument(buffer) abort
     let l:shell_type = ale#handlers#sh#GetShellType(a:buffer)
 
     if !empty(l:shell_type)
+        " Use the dash dialect for /bin/ash, etc.
+        if l:shell_type is# 'ash'
+            return 'dash'
+        endif
+
         return l:shell_type
     endif
 
     " If there's no hashbang, try using Vim's buffer variables.
-    if get(b:, 'is_bash')
+    if getbufvar(a:buffer, 'is_bash', 0)
         return 'bash'
-    elseif get(b:, 'is_sh')
+    elseif getbufvar(a:buffer, 'is_sh', 0)
         return 'sh'
-    elseif get(b:, 'is_kornshell')
+    elseif getbufvar(a:buffer, 'is_kornshell', 0)
         return 'ksh'
     endif