09_git.ipynb: Remove code copied to 10_checkpoint.ipynb

Remove unused imports from both 09_git.ipynb and 10_checkpoint.ipynb.

Replace `_repo_path_to_out_path()` with `_repo_graph_savefile()`, which in
turn uses `_savefile_name()` from the 'checkpoint' module (defined in
10_checkpoint.ipynb notebook), and `_repo_graph_name()`.  The process is
now split into stages, and it now uses common code and common conventions
for file naming.  Added more tests.

The `commit_graph()` function now stores the proper name of the commit
graph in the 'name' attribute of returned DiGraph, that is
'<repo name>-commit_graph' (via `_commit_graph_name()`).

In 10_checkpoint.ipynb notebook, use commit graph from Stolee blog posts
(`commit_graph()` from 'example_graphs' module, no relation to the
`commit_graph()` from 'commit_graph' module), instead of the commit
graph of the https://github.com/githubtraining/hellogitworld repo.

Add `guess_format()` function to 'checkpoint' module, and use it in
the `load_graph_df_from_file()`.

Author: jnareb