When --deep is not enabled, many dependencies are dicarded and

When --deep is not enabled, many dependencies are dicarded and
left out of the digraph. This patch prevents dependencies from
being discarded in some cases where the are needed in order to
optimize merge order. It also modifies the DepPriority.rebuild
attribute so that it only applies to build time dependencies.
This leads to better merge order in some cases when --deep is
not enabled. For example, `emerge xf86-input-keyboard
xorg-server` will now properly merge xorg-server before
xf86-input-keyboard (problem from bug #192254, comment #5).

svn path=/main/trunk/; revision=7797