amend: move createmarkers evaluation earlier

The value is used at multiple points in the function. Retrieving the
value in the middle of the transaction scope gives the false
impression that it has a single user. We move it at the start of the
function to clarify this.

mercurial/cmdutil.py

@@ -2454,6 +2454,7 b' def amend(ui, repo, commitfunc, old, ext'
 
     ui.note(_('amending changeset %s\n') % old)
     base = old.p1()
+    createmarkers = obsolete.isenabled(repo, obsolete.createmarkersopt)
 
     wlock = dsguard = lock = newid = None
     try:
@@ -2619,7 +2620,6 b' def amend(ui, repo, commitfunc, old, ext'
                         marks[bm] = newid
                     marks.recordchange(tr)
             #commit the whole amend process
-            createmarkers = obsolete.isenabled(repo, obsolete.createmarkersopt)
             if createmarkers and newid != old.node():
                 # mark the new changeset as successor of the rewritten one
                 new = repo[newid]