make commit-graph write configurable

[rremer]

The following configuration key is exposed, defaulting to the current behavior (backwards-compatible):

[git]
  ENABLE_COMMIT_GRAPH_WRITE = true

Provides a workaround for intermittent mirror-sync failures in #33239 , as well as significant optimization to sync time for very large repositories.

[rremer]

There's also this related comment in a different bug describing potential features in Gitea to handle stale lockfiles, but in reality I don't see this as clearly a Gitea problem to solve, as either:
a) it's a race condition where two processes (either managed by a golang Context or not) are trying to write a commit graph at once... arguably this could be handled better by Gitea
b) it's a stale lockfile because the git client itself exited nonzero, in which case the fix should be in git/git

I opened this PR as an alternative to deciding which (if either) to explore, because for the case of very large repositories the commit graph is very expensive to write (I have 4 repositories, each of which takes more than 1m20s to writeCommitGraph), and I haven't been able to prove the value of it for regular git fetches in my environments.

[rremer]

And a corresponding docs update if this is merged in: https://gitea.com/gitea/docs/pulls/264

[lunny]

The commit graph is intended for performance optimization, so I don’t think we should ignore it. Regarding #33239, it might have been caused by two mirror tasks running in parallel. I think we could add some locks to prevent to two mirror tasks running in the same repository.

[rremer]

The commit graph is intended for performance optimization, so I don’t think we should ignore it.

My request is that we make the option of disabling it available. For use-cases where there are very large repositories with extremely high commits rates (mono repos), commit-graphs don't actually provide any value, and actually impart a performance hit while generating the graph.

[lunny]

So that maybe it should be a per repository configuration item?