commit-graph: stop depending on `struct odb_source`

To read or write a commit graph we require access to the repository that
the graph should be read from or written for as well as the object
directory to derive its location. Instead of passing in these two bits
of information explicitly though, we pass in a `struct odb_source`,
which carries with it both of these pieces of information.

But ultimately, this is somewhat flawed as we now assume that the source
even has an on-disk representation, and furthermore we assume that the
commit graphs would live in ".git/objects/info". This is true for the
"files" backend, but it's not necessarily true for any other backend
that we may eventually want to introduce. So eventually, we'll want to
evolve the commit-graph subsystem to become agnostic of the backend's
layout and let the backend itself decide where to read a commit graph
from or where to write it to.

We're not there yet to do that switch, but the current design is already
causing issues for the intent of this patch series where we want to move
alternates into the "files" backend.

Convert the subsystem to take a repository plus an object directory path
instead. This unblocks moving around alternates, and it's also a step
in the right direction for moving commit graphs into the source in a
later series.

Note that we previously compared the `struct odb_source` pointers of two
commit graphs to figure out whether they were located in the same object
directory, whereas we now have to compare their paths. Callers may pass
in those paths in different forms though, for example relative to the
current working directory or as absolute paths. Convert the object
directory into an absolute path both when loading and when writing
commit graphs so that the comparisons become more robust.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>