merge: use constants for merge state record types
merge.py is using multiple discrete sets of 1 and 2 letter constants
to define types and behavior. To the uninitiated, the code is very
difficult to reason about. I didn't even realize there were multiple
sets of constants in play initially!
We begin our sanity injection with merge state records. The record
types (which are serialized to disk) are now defined in RECORD_*
constants.
Differential Revision: https://phab.mercurial-scm.org/D2698