Comparing two branches yields different diffs

DORSETRIGS

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06-10-2024

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Git Gotchas: Why Comparing Branches Shows Different Diffs

Have you ever found yourself staring at two seemingly identical Git branches, only to be met with confusingly different diffs when you compare them? This seemingly paradoxical situation can leave you scratching your head and wondering what's going on. Let's dive into the reasons behind this behavior and explore solutions.

The Scenario:

Imagine you're working on a project with two branches, feature-a and feature-b. Both branches were created from the same base commit, and you've been diligently working on your features, making frequent commits. When you compare the two branches, you expect to see only the differences related to the unique changes you've made. However, you're surprised to find that the diffs include changes you're certain you didn't make in either branch.

The Original Code:

git checkout feature-a
git diff feature-b 

This command will display the diff between the feature-a and feature-b branches, potentially showing changes you didn't expect.

Understanding the Issue:

The culprit behind this discrepancy is often upstream changes, which are changes made in the main branch (or any branch your branches are based on) after you created your feature branches. These changes get incorporated into the feature-b branch if it has been updated with the latest changes from the main branch.

Example:

• You create feature-a and feature-b from the same commit in the main branch.
• Someone else makes changes to main and pushes those changes.
• You switch to feature-b and update it with the latest main changes.
• When you compare feature-a and feature-b, you'll see the diffs from the main branch changes even though you didn't make them on feature-a.

Solutions:

1. Rebase your feature branch: Rebase feature-a onto the latest main branch to incorporate the upstream changes. This will create a linear history that reflects all the changes, including those made in main.

   git checkout feature-a
   git rebase main
   

2. Merge the main branch into your feature branch: This approach adds the changes from main as a new commit on top of your existing work in feature-a. While it preserves the history, it might make it more challenging to track the specific changes you made.

   git checkout feature-a
   git merge main
   

3. Compare against a specific commit: Instead of comparing the branches directly, compare feature-a with the commit where feature-b was branched from. This will only show the changes you made specifically in feature-a without including the upstream changes.

   git checkout feature-a
   git diff <commit-hash-of-feature-b-branch-point>
   

Key Takeaways:

• Upstream changes are a common source of confusing diffs.
• Rebasing or merging your feature branch with the main branch helps to align your work with the latest changes.
• Comparing against the branch point provides a more accurate view of your individual changes.

Remember, understanding the underlying reasons for these seemingly strange behaviors will empower you to troubleshoot and resolve issues in your Git workflows. By applying the right techniques, you can navigate the intricacies of branching and ensure your changes are properly reflected in your codebase.