treemanifest: fix a "treeinmem" case
f2c5b9d48b29 (manifest: make treemanifestctx store the repo,
2016-10-18) broke most tests when run with treeinmem=True. The
treeinmem mode can not be enabled by the user, so this did not break
anything in practice, but it's useful to have it working for testing
the treemanifest code.
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