mq: make qprev return the previous applied patch (issue3245)

The current behaviour is to return the previous one in the series but at the
same time the implementation is buggy because it does not take guarded patches
in account.

hgext/mq.py

@@ -2217,7 +2217,7 b' def next(ui, repo, **opts):'
 
 @command("qprev", seriesopts, _('hg qprev [-s]'))
 def prev(ui, repo, **opts):
-    """print the name of the previous patch
+    """print the name of the previous applied patch
 
     Returns 0 on success."""
     q = repo.mq
@@ -2228,7 +2228,8 b' def prev(ui, repo, **opts):'
     if not l:
         ui.write(_("no patches applied\n"))
         return 1
-    q.qseries(repo, start=l - 2, length=1, status='A',
+    idx = q.series.index(q.applied[-2].name)
+    q.qseries(repo, start=idx, length=1, status='A',
               summary=opts.get('summary'))
 
 def setupheaderopts(ui, opts):

tests/test-mq-guards.t

@@ -172,6 +172,8 b' should push b.patch'
   $ hg qpush -a
   applying c.patch
   now at: c.patch
+  $ hg qprev
+  b.patch
 
 Used to be an issue with holes in the patch sequence
 So, put one hole on the base and ask for topmost patch.

tests/test-mq.t

@@ -74,7 +74,7 b' help'
    qnew          create a new patch
    qnext         print the name of the next pushable patch
    qpop          pop the current patch off the stack
-   qprev         print the name of the previous patch
+   qprev         print the name of the previous applied patch
    qpush         push the next patch onto the stack
    qqueue        manage multiple patch queues
    qrefresh      update the current patch