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Read the tutorial and find how you can commit some part of the file easily. Also, know about interactive add mode and portions of the file called hunks.
In this tutorial, you will get useful information about the simplest way of comparing the difference between the local and remote branches step by step.
The following tutorial shows two ways of formatting files in a commit. You can find a preferred and less preferred ways with the explanation in detail.
This tutorial provides two solutions to the question of previewing the actual merge output in Git. Read about peculiarities of git merge and a lot more.
Read this tutorial and know which commands are used to make it possible to show the changes which have been changed. Other methods are also discussed.