Context

During a migration from SVN to Git, I faced the following situation:

In SVN, when a branch is created, it contains a single commit being the whole codebase.

In Git, a branch is just a symbolic link (kind of) to a commit. So when a branch is created from master, the branch's history is the same as master's.

After migrating the SVN branches in the Git repository, each branch had a different starting point, which made things complicated when the branch had to be merged back in master (many conflicts of files existing in both branches).

So I wrote a script to recreate the branch to base them on the commit they've been created from.

Explainations

Get the branch's first commit

First, I had to find the branch's first commit:

BRANCH_FIRST_COMMIT=`git rev-list $BRANCH | tail -n 1`

Find the commit where BRANCH_FIRST_COMMIT comes from

Then, I had to find in the parent, when the branch has been created. For that, I looped in the target's rev-list starting from the last:

COMMIT=$TARGET
DIFF='1'
while [ ! -z "$DIFF" ]
do
    COMMIT=`git rev-list $COMMIT^ | head -n 1`
    DIFF=`git diff $BRANCH_FIRST_COMMIT $COMMIT | head -n 1`
done 

Recreate the branch

Once the commit had been found, I recreated the branch from this commit. I did that by creating a new branch and cherry-picking all the original branch's commits (except for the first one, which is the branch creation) in the newly created:

git branch tmp-$BRANCH $COMMIT
git checkout tmp-$BRANCH
git rev-list $BRANCH_FIRST_COMMIT..$BRANCH | tac | awk 'NR > 0 { print }' | xargs -I [] sh -c "git cherry-pick []" 

Replace the branches

Finally, I replaced the original branch by the newly created:

git checkout $BRANCH
git branch -m $BRANCH-old
git checkout tmp-$BRANCH
git branch -m $BRANCH 

Global version

The whole script can be found at https://gist.github.com/padawin/9899f82913d237c1fd5c