define "checkins" and "commits"

[hbernard-keltechsl]

Defined the terms "checkins" and "commits", which are helpful for users to understand Figures 4, 5 and 6 in Section 1.3

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

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