Polish branches chapter

levels/branches/checkout-commit

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+title = Moving through time
+cards = checkout commit-auto
+
+[description]
+
+The yellow boxes are frozen points in time, we call them "commits"! You can travel between them using the "checkout" card! (Try it!)
+
+The grey panel below shows your current environment - click on an object to inspect or modify it!
+
+Can you find out what happened here? Then, while on the latest commit, fix the problem, and make a new commit!
+
+[setup]
+
+echo "This piggy bank belongs to the big sister.
+It contains 10 coins." > piggy_bank
+git add .
+git commit -m "The beginning"
+
+echo "A young girl with brown, curly hair." > little_sister
+git add .
+git commit -m "Little sister comes in"
+
+echo "Has 10 coins." >> little_sister
+echo "This piggy bank belongs to the big sister.
+It is empty." > piggy_bank
+git add .
+git commit -m "Little sister does something"
+
+git checkout HEAD^^
+git branch -df main
+
+[win]
+
+# Restore sisterly peace.
+{ git show HEAD:piggy_bank | grep "10 coins"; } && { git show HEAD:little_sister | grep -v "10 coins"; } && { git rev-parse HEAD^^^; }
+
+[congrats]
+
+Wonderful! Now that you're getting familiar with the time machine, let's look at some more complicated situations...