First version of an "index" chapter

levels/index/commit-a

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+title = Make a commit, but faster!
+cards = add reset checkout commit commit-a
+
+[description]
+
+There is a time-saving trick, where instead of a plain `git commit`, you can use
+
+    git commit -a
+
+This will automatically add all changes you made to local files! Very convenient.
+
+[setup]
+
+echo a > a
+echo b > b
+echo c > c
+git add .
+git commit -m "Initial commit"
+echo x > a
+echo x > b
+echo x > c
+
+[win]
+
+# Make a commit where all files contain "x".
+test "$(git show main:a)" = x && test "$(git show main:b)" = x && test "$(git show main:c)" = x