Merged from trunk 5995:6021

Merged from trunk 5995:6021

For bug #167450, optimize merge order by delaying the merge of root
nodes.

For bug #166880, always display the merge list (allowing --tree to be
obeyed in any case).

For bug #166880, always display the merge list header.

When calling tar, always exclud . in order to preserver
PORTAGE_WORKDIR_MODE.  Thanks to Brian Harring in bug #167544.

Don't exclude . since that excludes the entire contents of some tar
files.

For bug #166564, make a circular dependency panic more user friendly by
displaying the USE flags that are enabled on nodes that are part of
dependency cycles and display a note hinting that circular dependencies
can often be avoided by temporarily disabling USE flags.

Automatically enable --tree mode when displaying nodes with circular
deps.

For bug #167450, optimize leaf node selection by ordering nodes from
highest to lowest overall reference count.

Make digraph._merge_order_bias() operate on a single digraph that's
passed in.

Optimize order for --tree display of circular dependencies.

As requested by vapier, don't show the merge list in --quiet mode unless
it's explicitly requested via --ask or --pretend.

For bug #167795, try avoid the population routine when possible, so that
FEATURES=buildpkg doesn't always force population.

Raise a CommandNotFound exception when necessary, to ease
troubleshooting.  Thanks to marienz for reporting.

svn path=/main/branches/prefix/; revision=6080