lib/commit: Fix EBADF with GENERATE_SIZES option for commit

lib/commit: Fix EBADF with GENERATE_SIZES option for commit

Regression from previous tmpfile refactoring; unfortunately
the `OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES` option
only has coverage via gjs currently.

Might expose it via the cmdline in a later option, but in the big picture the
idea was that this data is better kept in static deltas.

Bug: https://github.com/ostreedev/ostree/issues/1014
Forwarded: https://github.com/ostreedev/ostree/pull/1016

Gbp-Pq: Name lib-commit-Fix-EBADF-with-GENERATE_SIZES-option-for-commi.patch