metadataonlyctx: don't crash when reusing the manifest with deletions This was originally fixed by Mateusz Kwapich for the `metaedit` command in fb-hgext with a test for the `metaedit` command. It didn't get upstreamed because `metaedit` was not in core. This patch fixes the crash and adds a test about `metadataonlyctx` to avoid future regressions. Differential Revision: https://phab.mercurial-scm.org/D550