overlayworkingctx: add `_checkexist(path)` This is in preparation to switch this class to inheriting (and being based off a) `commitctx` instead of a `workingctx`. `filectx` has no `exists` function, so this is how we'll fall back in that case. Differential Revision: https://phab.mercurial-scm.org/D1237

overlayworkingctx: fix a bad reference to `self._path` Differential Revision: https://phab.mercurial-scm.org/D1236

overlayworkingctx: track copy information in the context We can no longer do so in the dirstate, so add the functions to do so here. Differential Revision: https://phab.mercurial-scm.org/D1235

rebaseruntime: raise InMemoryMergeConflictsError on merge conflicts Differential Revision: https://phab.mercurial-scm.org/D1234

rebase: do not bail on uncomitted changes if rebasing in-memory Differential Revision: https://phab.mercurial-scm.org/D1233

rebase: add the --inmemory option flag; assign a wctx object for the rebase In the future, the --inmemory flag might be deprecated in favor of something more intelligent (for example, always rebasing in-memory if the working copy parent isn't in the rebaseset). But we might keep it as a way to explicitly force IMM on or off. Differential Revision: https://phab.mercurial-scm.org/D1232

overlayworkingctx: move _wrappedctx out of the constructor With rebase, we will be setting the _wrappedctx at a different point from the wctx construction (somewhat later, and possibly several times). Move it to a public function. Differential Revision: https://phab.mercurial-scm.org/D1231