A.09_git_explore.ipynb: Remove duplication, fix, expand

Main body of this change is removing accidental duplication, probably
caused by accidentally copying the contents from 09_git.ipynb a second
time.

Fix missing imports, replace non-portable `!dir` with pure-Python
solution, add some more descriptions (including new information).

The commit graph is created now from `git log --branches`, not
`git log --all` which for repositories cloned from GitHub includes pull
requests -- some data is left from earlier version (this should be noted
in comments in Markdown cells).

Author: jnareb