With git, you can stage line by line, file by file. You don’t need to commit everything at once. Also, you can add the lines you want in to staging, then use the stash feature to temporarily save lines that you don’t want in the commit. This allow you to test the commit without all of the extra lines you don’t want.

I would highly suggest reading Pro Git [0], a well written book on not only how to use git, but also the best practices, common workflows, and some of the internals in to how git works. Many refer to it as the essential book on git.

[0]: https://git-scm.com/book/en/v2