git: Use switch instead of checkout if available

For git versions that support it, the aliases will use the newer
`switch` instruction instead of `checkout` for its use cases.
Creating a new branch through `gcb` is for example done by
`git switch -c` instead of the older `git checkout -b`.

Additionally streamlined git version checking to be a little faster and
to unify its approach on posix sh, bash and zsh instead of utilizing
individual checking functions.

git/.config/zsh/alias.d/git-stash-push.sh

@@ -1,15 +0,0 @@
-#!/usr/bin/env zsh
-# shellcheck shell=sh
-
-if ! exist git; then
-    return 1
-fi
-
-# git alias
-# if git is at least version 2.13, we can use git stash push
-autoload -Uz is-at-least
-if is-at-least 2.13 "$(git --version 2>/dev/null | awk '{print $3}')"; then
-    alias gsta='git stash push'
-else
-    alias gsta='git stash save'
-fi