Fix debugging output for infinite backjumps.

Previously, a setting of `--max-backjumps=-1` was reported in
the debug output as `0`. But `0` has a very different meaning,
and that is confusing.

cabal-install/Distribution/Client/Dependency.hs

@@ -168,8 +168,8 @@ showDepResolverParams p =
   ++ "\navoid reinstalls: "  ++ show (depResolverAvoidReinstalls   p)
   ++ "\nshadow packages: "   ++ show (depResolverShadowPkgs        p)
   ++ "\nstrong flags: "      ++ show (depResolverStrongFlags       p)
-  ++ "\nmax backjumps: "     ++ show (fromMaybe 0 $
-                                      depResolverMaxBackjumps      p)
+  ++ "\nmax backjumps: "     ++ maybe "infinite" show
+                                     (depResolverMaxBackjumps      p)
   where
     showLabeledConstraint :: LabeledPackageConstraint -> String
     showLabeledConstraint (LabeledPackageConstraint pc src) =