mq: do not call ui.username unless it is necessary

After changeset c63c336ee2f7, ui.username will abort by default if no
username is set. It is therefore wrong to call ui.username when a
username was provided on the command line.

hgext/mq.py

@@ -1914,11 +1914,10 b' def prev(ui, repo, **opts):'
                      summary=opts.get('summary'))
 
 def setupheaderopts(ui, opts):
-    def do(opt, val):
-        if not opts[opt] and opts['current' + opt]:
-            opts[opt] = val
-    do('user', ui.username())
-    do('date', "%d %d" % util.makedate())
+    if not opts.get('user') and opts.get('currentuser'):
+        opts['user'] = ui.username()
+    if not opts.get('date') and opts.get('currentdate'):
+        opts['date'] = "%d %d" % util.makedate()
 
 def new(ui, repo, patch, *args, **opts):
     """create a new patch

tests/test-mq-qnew

@@ -54,3 +54,7 b' echo b >> b'
 hg st
 hg qnew -g -f p
 cat ../.hg/patches/p
+
+echo '% qnew -u with no username configured'
+HGUSER= hg qnew -u blue red
+cat ../.hg/patches/red

tests/test-mq-qnew.out

@@ -32,3 +32,6 b' diff --git a/d/b b/d/b'
 @@ -1,1 +1,2 @@
  b
 +b
+% qnew -u with no username configured
+From: blue
+