mergestate: merge `preresolve()` into `resolve()` The separation between `preresolve()` and `resolve()` was something added for the "merge driver" feature. Since we removed that in 32ce4cbaec4b (mergedriver: delete it, 2020-09-17), we don't need the separation anymore. More cleanups will follow. Differential Revision: https://phab.mercurial-scm.org/D11855