I have two projects. One is the "official" project and the second is a light modification (some files added). I created new branch and I put new files to them. But in during development some files common to both branches is changed.
How do I commit only these files?
I suppose you want to commit the changes to one branch and then make those changes visible in the other branch. In git you should have no changes on top of HEAD when changing branches.
You commit only the changed files by:
git commit [some files]
Or if you are sure that you have a clean staging area you can
git add [some files] # add [some files] to staging area
git add [some more files] # add [some more files] to staging area
git commit # commit [some files] and [some more files]
If you want to make that commit available on both branches you do
git stash # remove all changes from HEAD and save them somewhere else
git checkout <other-project> # change branches
git cherry-pick <commit-id> # pick a commit from ANY branch and apply it to the current
git checkout <first-project> # change to the other branch
git stash pop # restore all changes again