Pfedit: avoid Undoing too much

 It seems that we can currently do a successful Undo (but not two!)
 on a freshly started Lemma. Let's add a depth check on Pfedit.undo
 to avoid that. This way, we cannot Undo to depth < 1 (with 1 being
 the initial depth of a lemma).

 Simplier implementation of Pfedit.restart : it is Pfedit.undo_todepth 1.

 This extra initial step in the Undo Stack has probably something
 to see with the Lemma argument introduction. For instance, before this
 patch we had:

 Lemma test n : n+0=n.
 Restart. (* now the goal is universally quantified ! *)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@15075 85f007b7-540e-0410-9357-904b9bb8a0f7

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