Where am I? * (no branch)

Answer 1

It doesn't look like you have a branch called mybranch. If you want to create a branch that points to the current head, use git branch foo.

If you want one that points to another ref, use git checkout -b newbranch <ref>

Answer 2

The git checkout man page does mention, for the branch name argument:

<branch>

Branch to checkout;

• if it refers to a branch (i.e., a name that, when prepended with "refs/heads/", is a valid ref), then that branch is checked out.
• Otherwise, if it refers to a valid commit, your HEAD becomes "detached" and you are no longer on any branch.

So instead of having checked out a branch name, you must have checked out a tag name (valid commit), making your HEAD a detached one.