fetch: avoid locking issues between fetch.jobs/fetch.writeCommitGraph

fetch: avoid locking issues between fetch.jobs/fetch.writeCommitGraph

When both `fetch.jobs` and `fetch.writeCommitGraph` is set, we currently
try to write the commit graph in each of the concurrent fetch jobs,
which frequently leads to error messages like this one:

fatal: Unable to create '.../.git/objects/info/commit-graphs/commit-graph-chain.lock': File exists.

Let's avoid this by holding off from writing the commit graph until all
fetch jobs are done.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>