09_git.ipynb: Fix error making CI to fail

The notebook didn't ensure that the `repos/` subdirectory existed by
creating it before trying to store git.git repository in it.

This made other commands to fail non-fatally (because they shell out),
and the error message occured much later in the notebook, when trying
to draw histogram of 'level' of nodes in the commit graph.

There was no problem in local installation, because 'repos/' directory
existed there.

Author: jnareb

09_git.ipynb

@@ -139,6 +139,32 @@
     "- sparse: 14.9s - 19.4s"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Prepare the directory to store repositories (note: `mkdir -d <directory>` should show no error if `<directory>` exists.  \n",
+    "Clone example repository (for exploration)."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "A subdirectory or file repos already exists.\n",
+      "Error occurred while processing: repos.\n"
+     ]
+    }
+   ],
+   "source": [
+    "!mkdir -p repos"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -157,13 +183,28 @@
     "!git -C repos clone --mirror --filter=tree:0 --quiet https://github.com/git/git.git"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Extract the history of the project"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
    "metadata": {},
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "A subdirectory or file datasets already exists.\n"
+     ]
+    }
+   ],
    "source": [
-    "!mkdir datasets"
+    "!mkdir -p datasets"
    ]
   },
   {