git - Remove spurious commit parent pointer

Question

I imported a Bazaar repository into Git (using git bzr), but the resulting repository contains a spurious commit parent link:

Notice that the commit tagged 1.02-6 is based off the 1.02-3 commit, but 1.02-1 is unnecessarily also marked as a parent. (Note: All the commits in this part of the repo are tagged; there are no commits between the ones shown.)

I have tried rebasing in several ways (on the master branch: git rebase 1.02-3, git rebase -i upstream-1.02, git rebase --onto 1.02-1 1.02-3, git rebase --root upstream-1.02 --onto=other_branch), but in each case it fails with a merge conflict. These seem to be attempting more than is necessary; the history is correct except for an extra parent pointer being recorded in the commit tagged 1.02-6.

How do you remove the link in order to linearize the history? Is there a better way than manually cherry-picking all the commits in sequence?

Answer 1

The easiest way to do this (in git >= 1.6.5) is to use:

git replace --edit <sha>

and remove/add/change the Parent: lines.

Once you are happy the change is right, you can rewrite the commits to make the change permanent:

git filter-branch --tag-name-filter cat -- --all

In some cases it'll be noticeably quicker to only rewrite the commits involved and not the full history (thanks to Michael for mentioning this in the comments); e.g. to rewrite only commits on the current branch:

git filter-branch --tag-name-filter cat -- <new parent sha>..head

If you're not sure, use --all, otherwise you risk ending up with other branches/tags still referencing the temporary replacement object.