How do I commit only some files?

You can commit some updated files, like this:

git commit file1 file2 file5 -m "commit message"

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

Get a list of files you want to commit

$ git status

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

modified:   file1
modified:   file2
modified:   file3
modified:   file4

Add the files to staging

$ git add file1 file2

Check to see what you are committing

$ git status

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

    modified:   file1
    modified:   file2

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

    modified:   file3
    modified:   file4

Commit the files with a commit message

$ git commit -m "Fixed files 1 and 2"

If you accidentally commit the wrong files

$ git reset --soft HEAD~1

If you want to unstage the files and start over

$ git reset

Unstaged changes after reset:
M file1
M file2
M file3
M file4