install_monitor: handle submodules

describes the options, but in fact ignore them for now

src/zc/buildout/install_monitor.py

@@ -66,7 +66,18 @@ class GitInstallMonitor(object):
     def _record_changes(self, message):
         start = time.time()
         self._logger.debug("Adding %s to git, this may take some time", self._work_tree)
-        if self._git("status", "--porcelain"):
+        # One problem is that buildout directory contain git repositories, wich `git add` below
+        # will treat as submodules.
+        # I see several options:
+        #  * add the content of these git repositories as normal files using plumbing commands (see
+        #     https://www.git-scm.com/book/en/v2/Git-Internals-Git-Objects)
+        #  * manage them as submodules (see https://stackoverflow.com/a/4162672 https://stackoverflow.com/a/4185579
+        #     because git add does not seem to make a managed submodule )
+        #  * just ignore them.
+        #
+        # for now, they are ignored.
+
+        if self._git("status", "--ignore-submodules", "--porcelain"):
             self._git("add", self._work_tree)
             self._git("commit", "-m", message)
             self._logger.debug("Added %s to git in %.2f seconds", self._work_tree, time.time() - start)