Fix flake8 cd logic for invalid options

ale_linters/python/flake8.vim

@@ -40,27 +40,23 @@ endfunction
 
 function! ale_linters#python#flake8#GetCdString(buffer) abort
     let l:change_directory = ale#Var(a:buffer, 'python_flake8_change_directory')
+    let l:cd_string = ''
 
-    " map legacy options to new ones
-    if l:change_directory is# 1
-        let l:change_directory = 'file'
-    elseif l:change_directory is# 0
-        let l:change_directory = 'off'
+    if l:change_directory is# 'project'
+        let l:project_root = ale#python#FindProjectRootIni(a:buffer)
+
+        if !empty(l:project_root)
+            let l:cd_string = ale#path#CdString(l:project_root)
+        endif
     endif
 
-    if l:change_directory is# 'file'
-        return ale#path#BufferCdString(a:buffer)
-    elseif l:change_directory is# 'off'
-        return ''
+    if (l:change_directory is# 'project' && empty(l:cd_string))
+    \|| l:change_directory is# 1
+    \|| l:change_directory is# 'file'
+        let l:cd_string = ale#path#BufferCdString(a:buffer)
     endif
 
-    let l:project_root = ale#python#FindProjectRootIni(a:buffer)
-
-    if !empty(l:project_root)
-        return ale#path#CdString(l:project_root)
-    endif
-
-    return ale#path#BufferCdString(a:buffer)
+    return l:cd_string
 endfunction
 
 function! ale_linters#python#flake8#GetCommand(buffer, version) abort

test/command_callback/test_flake8_command_callback.vader

@@ -41,6 +41,19 @@ Execute(The option for disabling changing directories should work):
   \ ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
   \]
 
+  let g:ale_python_flake8_change_directory = 0
+
+  AssertLinter 'flake8', [
+  \ ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
+  \]
+
+  " Invalid options should be considered the same as turning the setting off.
+  let g:ale_python_flake8_change_directory = 'xxx'
+
+  AssertLinter 'flake8', [
+  \ ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
+  \]
+
 Execute(The option for changing directory to project root should work):
   silent execute 'file ' . fnameescape(g:dir . '/python_paths/namespace_package_tox/namespace/foo/bar.py')
 
@@ -60,6 +73,13 @@ Execute(The option for changing directory to file dir should work):
   \   . ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
   \]
 
+  let g:ale_python_flake8_change_directory = 1
+
+  AssertLinter 'flake8', [
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
+  \]
+
 Execute(The flake8 command callback should let you set options):
   let g:ale_python_flake8_options = '--some-option'