Stash changes to specific files

I know that file2.cpp, file2.h, and file3.cpp have been modified with content (i.e., not just formatted).
I want to stash the changes to these three files and then checkout an old revision, so that I can reapply the changes to these files after.

With Git 2.13 (Q2 2017), git stash will have officially a way to stash changes for specific files with

git stash push [--] [<pathspec>...]

See commit 9e14090, commit 1ada502, commit df6bba0 (28 Feb 2017), and commit 9ca6326, commit 6f5ccd4, commit f5727e2 (19 Feb 2017) by Thomas Gummerer (tgummerer).
^{(Merged by Junio C Hamano -- gitster -- in commit 44c3f09, 10 Mar 2017)}

As now documented:

For quickly making a snapshot, you can omit "push".
In this mode, non-option arguments are not allowed to prevent a misspelled subcommand from making an unwanted stash.
The two exceptions to this are stash -p which acts as alias for stash push -p and pathspecs, which are allowed after a double hyphen -- for disambiguation.

When pathspec is given to 'git stash push', the new stash records the modified states only for the files that match the pathspec.
The index entries and working tree files are then rolled back to the state in HEAD only for these files, too, leaving files that do not match the pathspec intact.

Note, as pointed out by medmunds in the comments, that git stash would use paths relative to the root folder of the git repo.

---

And with Git 2.26 (Q2 2020), "git rm" and "git stash" learns the new "--pathspec-from-file" option.

If you have a list of files to stash in filesToStash.txt, then this is enough:

git stash --pathspec-from-file=filesToStash.txt` is enough.

A nice option is using the interactive stash mode.

git stash --patch

It works mostly like the interactive add mode: you are going to be presented with a series of diffs showing the changes you have in your working tree and you have to choose which files (or only certain parts of a file!) you want to stash, the rest will be left intact.

From man git-stash:

With --patch, you can interactively select hunks from the diff between HEAD and the working tree to be stashed. The stash entry is constructed such that its index state is the same as the index state of your repository, and its worktree contains only the changes you selected interactively. The selected changes are then rolled back from your worktree. See the "Interactive Mode" section of git-add(1) to learn how to operate the --patch mode.

In your case, you will be able to see formatting-only hunks and stash them individually, without loosing your meaningful changes.

You can add the files with changes you want to keep, then stash the rest of the files and clear the stash:

git add file2.cpp file2.h file3.cpp
git stash --keep-index

At this point, you've stashed your unwanted changes. If you'd like to permanently get rid of them, run:

git stash drop

Now you have file2.cpp, file2.h, and file3.cpp staged for commit. If you then want to stash these files (and not commit them):

git reset
git stash

Now you'll be at your previous commit, with only those three files stashed.

Update:

Git 2.13 and later includes a more direct way to stash specific files with git stash push, as VonC explains in his answer.

You can also use git stash -p. This way you can select which hunks should be added to stash, whole files can be selected as well.

You'll be prompted with a few actions for each hunk:

y - stash this hunk
n - do not stash this hunk
q - quit; do not stash this hunk or any of the remaining ones
a - stash this hunk and all later hunks in the file
d - do not stash 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
? - print help