error: add InMemoryMergeConflictsError

We'll raise this exception in the merge code, and in-memory users like rebase
can catch it and retry without IMM.

Differential Revision: https://phab.mercurial-scm.org/D1210

mercurial/error.py

@@ -301,3 +301,7 @@ class CorruptedState(Exception):
 
 class PeerTransportError(Abort):
     """Transport-level I/O error when communicating with a peer repo."""
+
+class InMemoryMergeConflictsError(Exception):
+    """Exception raised when merge conflicts arose during an in-memory merge."""
+    __bytes__ = _tobytes