dbt deps does not handle non-master default branch

Steps to reproduce

1. Add a package using a the git syntax. The package must use a default branch other than master. In this example, the default branch is main (this package is currently an internal package but will be public soon)

$ cat packages.yml
packages:
  - git: [e-mail]:fishtown-analytics/dbt-artifacts.git
    

1. Run dbt deps

$ dbt deps
Running with dbt=0.19.0
Encountered an error:
Error checking out spec='master' for repo [e-mail]:fishtown-analytics/dbt-artifacts.git
fatal: couldn't find remote ref master

Expected behavior

Package should install from the default branch, main and warn about an unpinned package

If I do specify revision: main, I also expect to get an unpinned error, however currently no warning is shown.

dbt version

0.19.0 😃

Additional context

Relevant code

Going to be more common since GitHub updated their default branch name.

[VasiliiSurov]

@jtcohen6 , @clrcrl I want to pick up this task if nobody minds

Here is my ideas:

1. Add new method to client/git.py default_branch(repo) that will get the value of default branch from the git configuration using CLI git remote show [repository]
2. Modify deps/git.py@resolved instead of hard coded ‘master’ assign result value from default_branch https://github.com/fishtown-analytics/dbt/blob/c8f0469a44f84cfc7d48cb6b938c6cd27288feba/core/dbt/deps/git.py#L136
3. Modify deps/git.py@_fetch_metadata check self.revision for (‘master’, ‘main’) https://github.com/fishtown-analytics/dbt/blob/c8f0469a44f84cfc7d48cb6b938c6cd27288feba/core/dbt/deps/git.py#L75

And one more possible change that is out of the scope but is related

1. Modify clients/git.py@checkout instead of hard coded ‘master’ assign result value from the default_branch https://github.com/fishtown-analytics/dbt/blob/c8f0469a44f84cfc7d48cb6b938c6cd27288feba/core/dbt/clients/git.py#L55

[kwigley]

💡 just jotting down my thoughts here, I haven’t tested this out. I know this applies to GitHub: after running git clone the default branch defined in GitHub will be checked out locally. When there is no version supplied for a package in packages.yml, we should probably:

• after cloning the package, either skip checking out anything or check out the current branch (maybe even use HEAD here?)
• ensure unpinned warning is displayed

There should be no reference to master after these changed.

When the version supplied for a package is a branch name, we could:

• check if the version points to a branch as opposed to a tag and display an appropriate warning.

I apologize if this is repeating or different than was is mentioned above! I think the main issue here is git has no notion of default branch, so git clone is the important piece to get this info from.