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