How do I compare different branches in Visual Studio Code 1.9?

Is it possible?

Update: As of November, 2020, Gitlens appears within VSCode's builtin Source Control Panel

I would recommend to use: Git Lens.

Use the Git History Diff plugin for easy side-by-side branch diffing:

https://marketplace.visualstudio.com/items?itemName=huizhou.githd

Visit the link above and scroll down to the animated GIF image titled Diff Branch. You'll see you can easily pick any branch and do side-by-side comparison with the branch you are on! It is like getting a preview of what you will see in the GitHub Pull Request. For other Git stuff I prefer Visual Studio Code's built-in functionality or Git Lens as others have mentioned.

However, the above plugin is outstanding for doing branch diffing (i.e., for those doing a rebase Git flow and need to preview before a force push up to a GitHub PR).

UPDATE

Now it's available:

https://marketplace.visualstudio.com/items?itemName=donjayamanne.githistory

Until now it isn't supported, but you can follow the thread for it: GitHub

It's now possible by using the githistory extension.

Here's a small trick though: You can compare the latest commits from each branch and that would be the same as comparing two branches side by side or creating a PR.

Here's how to do that using githistory extension:

1. Open githistory
2. Pick the latest commit from your current branch by clicking on "Git Commit Icon" → (Usually it should be latest commit it the list). From the opened dropdown menu click on "Select this commit".
3. Pick the latest commit from the branch you want to compare to by clicking "Git Commit Icon".
4. As a result, the dropdown should appear with a few options → Select the last option that says "Compare with SHA" and you'll see the diff.

In the 11.0.0 version released in November 2020, GitLens views are now by default all placed under the source control tab in VSCode, including the Search & Compare view which has the compare branches functionality:

It can be changed back to the side bar layout in GitLens settings:

If you just want to view the changes to a particular file between the working copy and a particular commit using GitLens, the currently accepted answer can make it difficult to find the file you're interested in if many files have changed between the versions.

Instead, go to the file explorer in the side bar and right click on the file, go to Open Changes > Open Changes with Revision... (or Open Changes with Branch or Tag...).