Add feature: ref parameter for install_gitlab()

[balthasars]

Specifying a ref is not possible for in install_gitlab(), even though the underlying functions do support it.
Have done a fork with a first version at https://github.com/balthasars/remotes and am happy to do a PR for it, unless there's a good reason on why this is not enabled.

@gaborcsardi If this feature makes sense, could we add another branch to the falsy project on Gitlab so I can add some tests for installing from another branch? Can't do this using a merge request myself due to lack of permissions.

Thanks for the great work on the package!

[gaborcsardi]

We are focusing on improving https://github.com/r-lib/pak which already has this feature, so I suggest you try to see if pak meets your needs.

[balthasars]

Okay, I didn't want to use pak for production given that it has an experimental badge, didn't realize remotes was on the way out.
Can pak install from a self-hosted Gitlab instance using OAUTH tokens? Only saw personal access tokens, but maybe it's an oversight on my part.

[gaborcsardi]

Can pak install from a self-hosted Gitlab instance using OAUTH tokens? Only saw personal access tokens, but maybe it's an oversight on my part.

IDK, but if remotes can, then probably pak can as well.