Fix checking out a branch on in existing repo

[heewa]

What does this PR do?

In the case where a git repo already exists:

• if a branch is intended to be checked out, reset to origin/<branch>
  • this mirrors the behavior of the repo not existing on these lines, a little further down
  • it avoids problems stemming from a local branch by that name already existing, which may not even be related, or could have diverged from origin
• remove the unnecessary code that rebases from origin, since we just directly check it out (which happens after a fetch updates it a few lines up)

What issues does this PR fix or reference?

When a user first runs bootstrap-salt.sh with a branch specified (without a checked out repo) works fine, but sets up HEAD in a detached state to origin/<branch>. Subsequent runs of the bootstrap script fail when it tries to git reset --hard <branch> , with err:

fatal: ambiguous argument '': unknown revision or path not in the working tree.

Previous Behavior

Remove this section if not relevant

New Behavior

Remove this section if not relevant