Access private Git repositories using HTTP Basic authentication

[rdctmeconomou]

This was adapted from #41.

Unfortunately, work on go-git/go-git#490 and go-git/go-git#491 has been abandoned, so I would like to propose adding HTTP Basic authentication support back into bashbrew. Compared to the original pull request, I corrected the imports, created missing environment variable mappings (BASHBREW_GIT_USERNAME and BASHBREW_GIT_PASSWORD), and patched code added since the original pull request.

You can test this locally by setting BASHBREW_GIT_USERNAME to your GitHub username and BASHBREW_GIT_PASSWORD to an access token, e.g., the output of the gh auth token command. I recommend against specifying any password/token on the command line given how visible that is to other unprivileged processes or to attackers with access to one's shell history.

To test these changes in a GitHub Actions workflow:

1. Create a private GitHub repository containing a copy of docker-library/hello-world.
2. Register an app in the organization, disable webhook functionality, and grant read-only repository permissions for “Contents”. After completing app registration, generate a private key and install the app in the organization, giving it access to the private repository created in step 1.
3. Create a private GitHub repository containing a copy of the file library/hello-world from the Docker Official Images library, changing GitRepo to that of the private repository created in step 1, and .github/workflows/ci.yaml (attached), altering the bashbrew fork reference on line 42 as needed. In the GitHub Actions configuration of the second repository, set the variable PRIVATE_REPOSITORY_ACCESS_APP_ID to the ID of the app created in step 2 and the secret PRIVATE_REPOSITORY_ACCESS_KEY to the app's private key.

When the workflow runs, you should see in the output something similar to the following:

+ bashbrew cat ./library/hello-world
Maintainers: Tianon Gravi <admwiggin@gmail.com> (@tianon), Joseph Ferguson <yosifkit@gmail.com> (@yosifkit)
GitRepo: https://github.com/ResearchDataCom/hello-world.git
GitCommit: 0b0efba82b82ace81ab2fb42d25116f9488e6cb4

Note the private repository listed on the third line.

(The reference to the de-vri-es/setup-git-credentials action is unnecessary and only included to show the more customary Git credential store configuration.)

[rdctmeconomou]

@Mazgis47, I would be grateful for your review, as well.