builder-main: Prevent deleting cwd or its parents with force-clean

Building in the current directory does not make sense and cwd
usually contains the manifest, files referenced in the manifest and
other files. Deleting all of them is destructive.

Also since state dir defaults to cwd/.flatpak-builder by default,
a deletion leads to the hung state described in the previous commit.

Fixes #475

Author: bbhtt

src/builder-main.c

@@ -865,10 +865,12 @@ main (int    argc,
               g_autoptr(GFile) state_dir = builder_context_get_state_dir (build_context);
 
               if (g_file_equal (app_dir, state_dir) ||
+                  g_file_equal (app_dir, cwd_dir) ||
+                  g_file_has_prefix (cwd_dir, app_dir) ||
                   g_file_has_prefix (state_dir, app_dir))
                 {
-                  g_printerr ("Refusing to delete state directory "
-                              "or its parents\n");
+                  g_printerr ("Refusing to delete current working directory, state directory "
+                              "or their parents\n");
                   return 1;
                 }