git credential helper docs & support

[aran]

Sapling appears to work just fine with the git credential helper protocol. For example I can configure the gcloud auth credential helper in ~/.gitconfig and sl clone works.

I can't find any reference to this functionality in the Sapling documentation. All the docs discussing similar features are specific to Github and gh even though the implementation appears to be more general and powerful.

It would be nice to configure a credential helper within the repository-specific {project}/.sl/config rather than globally. I'm not sure if this is documented or supported.

[quark-zju]

Good point. There is a .sl/git/config (which is considered as impl detail and is likely going to change) that can probably be edited to achieve what you want. Perhaps we should expose an API or config to allow editing Git configs.

[aran]

For bonus points, let me specify a credential helper inline with a flag to sl clone, use it for the clone operation, and configure the resulting checked out repository to use it going forward :)