merge: move definition of `partial` closer to where it's used

Differential Revision: https://phab.mercurial-scm.org/D7983

mercurial/merge.py

@@ -2288,13 +2288,6 b' def update('
                     ),
                 )
             )
-    # If we're doing a partial update, we need to skip updating
-    # the dirstate, so make a note of any partial-ness to the
-    # update here.
-    if matcher is None or matcher.always():
-        partial = False
-    else:
-        partial = True
     with repo.wlock():
         if wc is None:
             wc = repo[None]
@@ -2507,6 +2500,13 b' def update('
         ### apply phase
         if not branchmerge:  # just jump to the new rev
             fp1, fp2, xp1, xp2 = fp2, nullid, xp2, b''
+        # If we're doing a partial update, we need to skip updating
+        # the dirstate, so make a note of any partial-ness to the
+        # update here.
+        if matcher is None or matcher.always():
+            partial = False
+        else:
+            partial = True
         updatedirstate = not partial and not wc.isinmemory()
         if updatedirstate:
             repo.hook(b'preupdate', throw=True, parent1=xp1, parent2=xp2)