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

bin/pull-git-folder

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+pull-git-folder() {
+    if [[ -z "$1" ]]; then
+        echo '$1/branch is empty.'
+        exit
+    fi
+    if [[ -z "$2" ]]; then
+        echo '$2/path is empty.'
+        exit
+    fi
+
+    local git_folders=$(ls-git-folder "$2")
+    for git_folder in $git_folders; do
+        echo "Pull git folder $git_folder."
+        git -C "$git_folder" stash
+        local update=$(git config submodule."$git_folder".update)
+        if [[ -z "$update" ]]; then
+            git -C "$git_folder" pull origin "$1"
+        fi;
+    done
+}