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

context: make parents and text optional in metadataonlyctx The metadataonlyctx is to copy an existing context with some minor metadata changes. If the caller only wants to change "extra", or "user", ideally it does not have to read and pass "parents" and "text" information. This patch makes "parents" and "text" optionally to convenient callers. Differential Revision: https://phab.mercurial-scm.org/D548

test: add more obsmarker tests for pruning scenarios The obsfate output in cases of pruning is not ideal right now, add some tests so have these scenarios around.