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
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`