When I make changes to a file in Git, how can I commit only some of the changes?

For example, how could I commit only 15 lines out of 30 lines that have been changed in a file?

You can use:

git add --patch <filename>

or for short:

git add -p <filename>

Git will break down your file into what it thinks are sensible "hunks" (portions of the file). It will then prompt you with this question:

Stage this hunk [y,n,q,a,d,/,j,J,g,s,e,?]?

Here is a description of each option:

• y stage this hunk for the next commit
• n do not stage this hunk for the next commit
• q quit; do not stage this hunk or any of the remaining hunks
• a stage this hunk and all later hunks in the file
• d do not stage this hunk or any of the later hunks in the file
• g select a hunk to go to
• / search for a hunk matching the given regex
• j leave this hunk undecided, see next undecided hunk
• J leave this hunk undecided, see next hunk
• k leave this hunk undecided, see previous undecided hunk
• K leave this hunk undecided, see previous hunk
• s split the current hunk into smaller hunks
• e manually edit the current hunk
  • You can then edit the hunk manually by replacing +/- by # (thanks veksen)
• ? print hunk help

If the file is not in the repository yet, you can first do git add -N <filename>. Afterwards you can go on with git add -p <filename>.

Afterwards, you can use:

• git diff --staged to check that you staged the correct changes
• git reset -p to unstage mistakenly added hunks
• git commit -v to view your commit while you edit the commit message.

Note this is far different than the git format-patch command, whose purpose is to parse commit data into a .patch files.

Reference for future: Git Tools - Interactive Staging