How to save changes when in detached-head state?

Disclaimer: git isn't complicated, it's just versatile. Don't be scared off just because I've rambled into a long answer :)

You had:
master: a-b-c-d-e-f

and wanted to change c. You did:
* git checkout c (avoid checking out commits in future. Move the branch head instead)
* changed some files

You are in:

master: a-b-c-d-e-f
             \uncommitted-work,detached

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If you want to re-apply d-e-f on top of your changed "c"

(If you have pushed, people downstream will be have to recover from upstream rebase)

1. git stash .
2. git checkout master
3. git stash pop (resolve conflicts)
4. git stage .
5. git commit -m "temporary name for g"
6. (master: a-b-c-d-e-f-g)
7. git rebase c -i ("re-apply my current branch on to point c, and let me manipulate the commits interactively", i.e, re-parent (rebase) d-e-f onto a new c)
8. Follow guide to interactive rebase. You want to re-order g so it's after c, then change the rebase command from pick to fixup. dd to delete a line, P to place it, i to enter insert mode to type "fixup" then :wq to save and exit vim.
9. (master: a-b-c'-d'-e'-f', where c' is the result of you merging g and c during the rebase. d-e-f have become d'-e'-f' as their ancestry has changed so they're not the "same" commits as far as git is concerned, but their contents remain the same)

---

If you want to undo d-e-f (and rewrite history as if you never made them)

(If you have pushed, people downstream will be have to recover from upstream rebase) :

1. git stash .
2. git checkout master
3. (master: a-b-c-d-e-f, with stashed files originally based upon c)
4. git reset --hard c (discard all files and commits on master since c)
5. (master: a-b-c, with stashed files)
6. git stash pop (resolve conflicts)
7. (master: a-b-c-*)
8. git stage .
9. git commit -m "description of g"
10. (master: a-b-c-g)

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If you want to undo d-e-f (but keep them in history)

1. git stash
2. git revert --no-commit d
3. git revert --no-commit e
4. git revert --no-commit f
5. git push
6. git stash pop (will be no conflicts)
7. git stage .
8. git commit -m "Undo d-e-f in order to fix..."
9. git push

---

If you have git push d-e-f, and you want to keep them separate:

Sounds like your new changes are for a new branch off master. git branch <foo>.

Create a new branch from this commit and THEN do the commit:

git checkout -b <branchname>
git commit 

Assuming you already staged (i.e. git add myfile1 myfile2) your files.

to save your work in the detached head:

git checkout -b <new-branch-name-for-detached-head> <sha-of-the-detached-head>

and if you want to merge it to master:

git checkout master
git merge <new-branch-name-you-have-just-created>