commit-graph: Use the commit-graph in revwalks

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.

The callbacks are currently sparsely documented, making it really hard
to implement a new backend without taking a look at the existing
refdb_fs backend. Add documentation to make this task hopefully easier
to achieve.

cred: change enum to git_credential_t and GIT_CREDENTIAL_*

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.