Add branch and tag to git_install()

[rkrug]

I would suggest to add a ref argument to the function install_git(), similar to the one in the install_github() to enable the installation of tagged versions or branches from a git repository.

As pointed out by Winston Chang (Conversation "ref parameter for install_git?" on the devtools mailing list), -b (or --branch) could used in git clone:

At the moment install_git_single uses

request <- system2(
    git_binary_path,
    args = c('clone', '--depth', '1', '--no-hardlinks', git_url,  bundle),
    stdout = FALSE, stderr = FALSE
  )

to clone the git repo.

request <- system2(
    git_binary_path,
    args = c('clone', '--branch = \"', ref, '\" ', '--depth', '1', '--no-hardlinks', git_url,  bundle),
    stdout = FALSE, stderr = FALSE
  )

i.e. adding the --branch = ... parameter, should work as expected.

Cheers,

Rainer