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

Deleting the cwd leaves flatpak-builder in an inconsistent hung state
by default as rofiles-fuse tries to recurse indefinitely when
resolving paths:

```
statx(AT_FDCWD, "/home/bbhtt/test/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/rofiles/rofiles-lLitmd/.flatpak-builder/cache/config", AT_STATX_SYNC_AS_STAT|AT_SYMLINK_NOFOLLOW|AT_NO_AUTOMOUNT, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=67, ...}) = 0
[...]
```

And generally this leads to unnecessary data loss when the cwd
often contains manifest and other project files.

Fixes #475

Author: bbhtt

src/builder-main.c

@@ -862,6 +862,14 @@ main (int    argc,
         {
           if (opt_force_clean)
             {
+              if (g_file_equal (app_dir, cwd_dir) ||
+                  g_file_has_prefix (cwd_dir, app_dir))
+                {
+                  g_printerr ("Refusing to delete current working "
+                              "directory or its parents\n");
+                  return 1;
+                }
+
               g_print ("Emptying app dir '%s'\n", app_dir_path);
               if (!flatpak_rm_rf (app_dir, NULL, &error))
                 {