commit: Don't delete tmp/cache dir

We hold a fd open on this, and it's basically now expected
to be immortal.  Confer that status.

This was showing up in flatpak crashers, because we'd get
an unexpected errno.

(I didn't test this fixes the crasher, but it's clearly right)

https://bugzilla.redhat.com/show_bug.cgi?id=1347293

Closes: #476
Approved by: alexlarsson

src/libostree/ostree-repo-commit.c

@@ -1219,6 +1219,12 @@ cleanup_tmpdir (OstreeRepo        *self,
       if (dent == NULL)
         break;
 
+      /* Special case this; we create it when opening, and don't want
+       * to blow it away.
+       */
+      if (strcmp (dent->d_name, "cache") == 0)
+        continue;
+
       if (TEMP_FAILURE_RETRY (fstatat (dfd_iter.fd, dent->d_name, &stbuf, AT_SYMLINK_NOFOLLOW)) < 0)
         {
           if (errno == ENOENT) /* Did another cleanup win? */