tests: make unshelve tests more tricky - don't depend on size change One reason shelve and largefiles doesn't work could be rapidly changing standin files. Prove that shelve in general doesn't have problems with that.

shelve: introduce secret option for using fixed date for temporary commit Using a fixed date makes hashes stable and makes debugging simpler. The date and hashes of this changeset are normally not exposed.

mq: repo['.'] is not a wctx, repo[None] is The parameters passed to subrepo.submerge are confusing anyway.