If commit timestamps are off, we're more likely to hit a traversal
where the first path ends up traversing past the root commit of the tree.
If that happens, it's possible that the loop will complete before the second
path marks some of those final parents. This fix keeps track of the root
nodes that are encountered in the traversal, and verify that they are
properly marked.
In the best case, with accurate timestamps, the traversal will continue
to terminate when all the commits are STALE (parents of a merge-base), as
it did before. In the worst case, where one path makes a complete traversal
past a root commit, we will continue the loop until the root commit itself
is marked.
`revwalk.h:commit_lookup()` -> `git_revwalk__commit_lookup()`
and make `git_commit_list_parse()` do real error checking that
the item in the list is an actual commit object. Also fixed an
apparent typo in a test name.
Moved it into graph.{c,h} which i created for the new "graph"
functions namespace. Also adjusted the function prototype
to use `size_t` and `const git_oid *`.
