This change does a medium-size refactor of the git_commit_graph_file and
the interaction with the ODB. Now instead of the ODB owning a direct
reference to the git_commit_graph_file, there will be an intermediate
git_commit_graph. The main advantage of that is that now end users can
explicitly set a git_commit_graph that is eagerly checked for errors,
while still being able to lazily use the commit-graph in a regular ODB,
if the file is present.
This change introduces `git_commit_graph_entry_find()` and
`git_commit_graph_entry_parent()`. These two functions allow a much
faster lookup of commits by ID, since the ODB does not need to be
consulted, the commit object does not need to be inflated, and the
contents of the commit object do not need to be parsed.
Part of: #5757
This change is the first in a series to add support for git's
commit-graph. This should speed up commit graph traversals by avoiding
object parsing and allowing some operations to terminate earlier.
Part of: #5757
