Parallel submodules update (--jobs <n>)

[lukasz-mitka]

I'd like to request a new feature: parallel submodules update.

Why: speed up checkout process.

How: config option to allow specifying number of jobs for submodules update.

Docs:

• https://git-scm.com/docs/git-submodule#Documentation/git-submodule.txt---jobsltngt

Related:

• https://github.com/actions/checkout/issues/370#issuecomment-737375554
• https://github.com/actions/checkout/issues/370#issuecomment-1212053523