This change introduces a new API function
`git_graph_reachable_from_any()`, that answers the question whether a
commit is reachable from any of the provided commits through following
parent edges.
This function can take advantage of optimizations provided by the
existence of a `commit-graph` file, since it makes it faster to know
whether, given two commits X and Y, X cannot possibly be an reachable
from Y.
Part of: #5757
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 *`.
