checkout_branch: support "-" argument (#322)

git natively supports `-` as an argument to `git switch` and `git
checkout`. It is shorthand for `@{-1}`, which is a way to refer to the
last branch you were on.

forgit used to interpret `-` as a branch name, detect that it does not
exist yet and create a new one with this name, which does not work.

Add a check whether `-` is passed on the command line and do not create
a new branch in this case

bin/git-forgit

@@ -454,7 +454,7 @@ _forgit_checkout_branch() {
     _forgit_inside_work_tree || return 1
     # if called with arguments, check if branch exists, else create a new one
     if [[ $# -ne 0 ]]; then
-        if git show-branch "$@" &>/dev/null; then
+        if [[ "$*" == "-" ]] || git show-branch "$@" &>/dev/null; then
             git switch "$@"
         else
             git switch -c "$@"