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
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']