gitweb: include navigation for non-commit-ishes
It's possible to view a tree or blob directly by using the tree
or blob's hash or a tag that points directly to one (without
an intervening commit). In that case it does not make sense
to show 'commit', 'commitdiff' or 'tree' (in the case of a blob)
options in the navigation line, but things like 'summary' and
'log' still apply.
Modify the code to provide minimal navigation links in these
cases rather than nothing at all which was previously the case.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>