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

bin/clone-git-folder

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+clone-git-folder() {
+    if [[ -z "$1" ]]; then
+        echo '$1/path is empty.'
+        exit
+    fi
+
+    local git_folders=$(ls-git-folder "$1")
+    for git_folder in $git_folders; do
+
+        git_folder="${git_folder#./}"
+
+        local git_folder_path=$(git config --file .gitmodules --get submodule."$git_folder".path)
+        local git_folder_url=$(git config --file .gitmodules --get submodule."$git_folder".url)
+        local shallow=$(git config --file .gitmodules --get submodule."$git_folder".shallow)
+        local git_folder_branch=$(git config --file .gitmodules --get submodule."$git_folder".branch)
+
+        if [[ "$shallow" = "true" ]]; then
+            echo "Clone git folder $git_folder (shallow) with $git_folder_branch branch"
+            git -C "$git_folder" config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
+            git clone --depth 1 --no-single-branch "$git_folder_url" "$git_folder_path"
+        else
+            echo "Clone git folder $git_folder with $git_folder_branch branch"
+            git clone --branch="$git_folder_branch" "$git_folder_url" "$git_folder_path"
+        fi
+    done
+}