overlayworkingctx: invalidate the manifest cache when changing parents This fixes problems noticeable when rebasing several commits into one destination commit using ``--collapse``. The manifest cache needs to be cleared each time. Differential Revision: https://phab.mercurial-scm.org/D1244

overlayworkingctx: add `_compact()` Alas, presence of a key in the cache isn't sufficient evidence that the file is actually dirty. Differential Revision: https://phab.mercurial-scm.org/D1243

overlayworkingctx: add ``tomemctx()`` Differential Revision: https://phab.mercurial-scm.org/D1242

overlayworkingctx: add _auditconflicts to write() Alas, part of Mercurial's conflict detection (for file<->folder conflicts, for example) depends on the filesystem. We don't have the filesystem with IMM, so we have to run these checks ourselves. Differential Revision: https://phab.mercurial-scm.org/D1241

overlayworkingctx: inherit from committablectx instead of workingctx We should find a better name for this context at some point. (mutablememctx?) Differential Revision: https://phab.mercurial-scm.org/D1240

overlayworkingctx: add a no-op ``clearunknown()`` We no longer inherit ``workingctx``'s version, but we also don't need to do anything anymore. Differential Revision: https://phab.mercurial-scm.org/D1239

overlayworkingctx: add _manifest, files(), added(), removed(), modified() Differential Revision: https://phab.mercurial-scm.org/D1238

rebase: rerun a rebase on-disk if IMM merge conflicts arise Differential Revision: https://phab.mercurial-scm.org/D1249