Switching branches without touching the working tree?

You can do the following:

git checkout --detach
git reset --soft master
git checkout master

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Explanation:

If you are on the debug branch and would do git reset --soft master you would leave your working tree and index untouched and move to the commit master points to. The problem is, debug will be reset to this commit too. So your commits on debug are "lost" (well, not really, but they are not directly accessible anymore) and you are still on the debug branch.

To prevent git reset from moving debug but still setting your HEAD to the master commit, you first do git checkout --detach to point HEAD directly to your current commit (see man git-checkout, section "DETACHED HEAD"). Then you can do the reset without touching the debugbranch.

Now HEAD is pointing directly to the commit master points to, i.e. it is still detached. You can simply git checkout master to attach to master and are now ready to commit on the master branch.

Note that git checkout (by default and when no path is passed) only updates files that have been changed between the "source" and "target" commit and local modifications to the files in the working tree are kept. As both commits are the same in this case, no files in the working directory are touched.

This answer uses low-level "plumbing" commands. Be careful. If you prefer "porcelain" commands, go with this answer which produces the same results.

You can reset your head to point at master without changing the index or working tree with:

git symbolic-ref HEAD refs/heads/master

You should probably reset the index so that you can selectively apply your working tree changes, otherwise you may end up committing all the differences between master and the debug branch, which is probably a bad thing.

git reset

Once you've made the commit that you want to make you can return to your debug branch with:

git symbolic-ref HEAD refs/heads/debug-branch
git reset