merge: reorder some initialization to make more sense

This puts the closely related definitions of "pl", "p1", "p2", "pas"
close together, and moves the definition of "overwrite" away and
closer to where it's first used.

Differential Revision: https://phab.mercurial-scm.org/D6471

mercurial/merge.py

@@ -1977,13 +1977,10 b' def update(repo, node, branchmerge, forc'
             wc = repo[None]
         pl = wc.parents()
         p1 = pl[0]
+        p2 = repo[node]
         pas = [None]
         if ancestor is not None:
             pas = [repo[ancestor]]
-
-        overwrite = force and not branchmerge
-
-        p2 = repo[node]
         if pas[0] is None:
             if repo.ui.configlist('merge', 'preferancestor') == ['*']:
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
@@ -1993,6 +1990,7 b' def update(repo, node, branchmerge, forc'
 
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), bytes(p1), bytes(p2)
 
+        overwrite = force and not branchmerge
         ### check phase
         if not overwrite:
             if len(pl) > 1: