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 execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..