feat(git): add comprehensive git folder management commands and update documentation

bin/switch-git-folder

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+switch-git-folder() {
+    if [[ -z "$1" ]]; then
+        echo '$1/path is empty.'
+        exit
+    fi
+    if [[ -z "$2" ]]; then
+        echo '$2/path is empty.'
+        exit
+    fi
+
+    local git_branch="$1"
+    local git_folders=$(ls-git-folder "$2")
+    for git_folder in $git_folders; do
+
+        git_folder="${git_folder#./}"
+
+        local shallow=$(git config --file .gitmodules --get submodule."$git_folder".shallow)
+
+        if [[ "$shallow" = "true" ]]; then
+            echo "Fetch branch $git_branch (shallow) from origin and then switch."
+            git -C "$git_folder" fetch origin "$git_branch:$git_branch"
+            git -C "$git_folder" switch "$git_branch"
+        else
+            echo "Switch git folder branch to $git_branch: $git_folder"
+            git -C "$git_folder" stash
+            git -C "$git_folder" switch "$git_branch"
+        fi
+    done
+}