Question

Git filter-tree helpfully stored a backup in .git/refs/original/refs/heads/tmp. But I can’t find any instructions on how to restore from it. I haven’t touched the repository since then – I just changed my mind about the wisdom of that particular manipulation.

I’m guessing the answer is either a file copy or git update-ref, but these are pretty destructive operations, and I don’t want to get it wrong.

Answer 1

Here’s the thing to know, from which all else flows: except when doing git gc (well, and things git gc does as well), git never removes anything, it only ever adds new things.

Let’s say you have a branch that looks something like this:

* 9d2e82c handle another temporary failure mode
| * 2afb8a0 (resubmit) checkpoint: resubmit
| * 90dcfbb move definitinon of GRONK to sysdefs
|/  
* 1ba7847 treat XML login errors as temporary failures

(this is snipped and slightly modified output from git lola: http://blog.kfish.org/2010/04/git-lola.html). Now say I did something to my “resubmit” branch, like an incorrect filter-branch, so that I have two new commits on that branch (1234567 and 7654321, or whatever). I realize, oh no, I did something completely broken, and I want “resubmit” to go back to pointing to commit 2afb8a0, which is the original tip of that branch.

That commit is still in git, and will be for several months at least (longer because you have the user-visible branch-name original/refs/heads/tmp—try doing git log on it for instance, you’ll see it’s there). Find the commit ID—easy in your case, it’s named original/refs/heads/tmp—and make your branch point there. If you didn’t have a name, you could do git branch -d resubmit and then git branch resubmit 2afb8a0 (for my own example above), but you do, so:

git branch -D tmp
git branch -m original/refs/heads/tmp tmp

will delete your current tmp and rename the renamed one back to tmp.

(The new commits filter-branch recently added, that you decided you don’t like after all, will still be in your repository. Once you’ve deleted the user-visible-names they’ll expire in about 3 months, after which git gc will remove them.)