restore_git_history.sh

# Suppose I've pulled $CHILD out of $PARENT but lost the git history in the proccess.
# Let the last commit's sha1 that $CHILD was in $PARENT be denoted $LAST_COMMIT.
# Suppose further that I rename $CHILD, now I have $CHILD_OLD_NAME and $CHILD_NEW_NAME.
# If $CHILD was not renamed when pulled out you can use just $CHILD or set $CHILD_OLD_NAME and
# $CHILD_NEW_NAME to the same thing.
cd $PARENT
git co $LAST_COMMIT
git log --pretty=email --patch-with-stat --reverse --full-index --binary -- $CHILD_OLD_NAME > patch
cd ../$CHILD_NEW_NAME
git co master
git pull --rebase
git checkout --orphan newroot
git rm -rf .
git am < ../$PARENT/patch
git co master
git co -b test # For a dry run.
git rebase newroot # Figure out what conflicts are and how to resolve.
git co master # Now do it for real.
git rebase newroot
git branch -D test newroot
git push --force origin master