Fetching from multiple remotes into the same repository in parallel
had a bad interaction with the recent change to (optionally) update
the commit-graph after a fetch job finishes, as these parallel
fetches compete with each other.  Which has been corrected.

* js/fetch-multi-lockfix:
  fetch: avoid locking issues between fetch.jobs/fetch.writeCommitGraph
  fetch: add the command-line option `--write-commit-graph`