commit-graph: return the prepared commit graph from `prepare_commit_graph()`

When making use of commit graphs, one needs to first prepare them by
calling `prepare_commit_graph()`. Once that function was called and the
commit graph was prepared successfully, the caller is now expected to
access the graph directly via `struct object_database::commit_graph`.

In a subsequent change, we're going to move the commit graph pointer
from `struct object_database` into `struct odb_source`. With this
change, semantics will change so that we use the commit graph of the
first source that has one. Consequently, all callers that currently
deference the `commit_graph` pointer would now have to loop around the
list of sources to find the commit graph.

This would become quite unwieldy. So instead of shifting the burden onto
such callers, adapt `prepare_commit_graph()` to return the prepared
commit graph, if any. Like this, callers are expected to call that
function and then use the returned commit graph.

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

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