revision: create mark_trees_uninteresting_dense()

The sparse tree walk algorithm was created in d5d2e93 (revision:
implement sparse algorithm, 2019-01-16) and involves using the
mark_trees_uninteresting_sparse() method. This method takes a repository
and an oidset of tree IDs, some of which have the UNINTERESTING flag and
some of which do not.

Create a method that has an equivalent set of preconditions but uses a
"dense" walk (recursively visits all reachable trees, as long as they
have not previously been marked UNINTERESTING). This is an important
difference from mark_tree_uninteresting(), which short-circuits if the
given tree has the UNINTERESTING flag.

A use of this method will be added in a later change, with a condition
set whether the sparse or dense approach should be used.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee