Reconsider the use of `-B` for the `git checkout` of remote branches

[jtpio]

As noticed in #1352, the git checkout command run when clicking on a remote branch uses the -B flag, which can be quite disruptive as it implies forcing the switch to the branch: https://git-scm.com/docs/git-checkout#Documentation/git-checkout.txt--Bltnew-branchgt

jupyterlab-git/jupyterlab_git/git.py

Line 1141 in da4710f

cmd = ["git", "checkout", "-B", local_branchname, branchname]

For now this was mitigated in #1366 with the addition of a confirmation dialog, to warn users of potential data loss.

It could however be interesting to still be able to check out remote branches via the UI, and be able to switch back to a local branch, without losing data, and similar to the workflow available via the git command directly.

[gjmooney]

Playing around with this, modifying https://github.com/jupyterlab/jupyterlab-git/blob/main/jupyterlab_git/git.py#L1128 to be a normal git checkout <branch_name> for remote branches works the same as using the command line, the UI reflects being in a detached head state, and switching back to the local branch retains any changes as you would expect.

[jtpio]

For reference, this -B option was added in #1066 to fix #741.