git - How to create a new (and empty!) "root" branch?

Question

I would like to define a new "root" branch in this git repository. By "root" branch I mean a branch that is entirely independent of all the other branches in the repository¹.

Unfortunately, even the commit (let's call it A) at the very base of the repo's commit tree contains a lot of files (this was a repository that was initialized on an already fairly mature project).

This means that even if I gave A as the new branch's <start-point>, this new branch would not start from a "clean slate", but rather it would contain all the files that were committed in A.

Is there some way I can create a completely bare branch in this repository, with <start-point> as close to A as possible?

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¹BTW, this is not equivalent to creating a new repo. Separate repos would be less convenient for a lot of reasons.

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EDIT: OK, this is what I did, based on vcsjones' answer:

# save rev of the current earliest commit
OLDBASE=$(git rev-list --max-parents=0 HEAD)

# create a new orphan branch and switch to it
git checkout --orphan newbranch
# make sure it's empty
git rm -rf .

# create a new empty commit in the new branch, and
# save its rev in NEWBASE
git commit --allow-empty -m 'base commit (empty)'
NEWBASE=$(git rev-list HEAD)

# specify $NEWBASE as the new parent for $OLDBASE, and
# run filter-branch on the original branch
echo "$OLDBASE $NEWBASE" > .git/info/grafts
git checkout master
git filter-branch

# NOTE: this assumes that the original repo had only one
# branch; if not, a git-filter-branch -f <branch> command
# need to be run for each additional branch.

rm .git/info/grafts

Although this procedure is a bit involved, the end result is an empty base commit that can serve as the <start-point> for any new "clean-slate branch"; all I'd need to do then is

git checkout -b cleanslate $(git rev-list --max-parents=0 HEAD)

In the future I will always create new repositories like this:

git init
git commit --allow-empty -m 'base commit (empty)'

...so that the first commit is empty, and always available for starting a new independent branch. (This would be, I know, a very rarely needed facility, but it is quite effortless to make it readily available.)

Answer 1

Use the --orphan when creating the branch:

git checkout --orphan YourBranchName

This will create a new branch with zero commits on it, however all of your files will be staged. At that point you could just remove them.
("remove them": A git reset --hard will empty the index, leaving you with an empty working tree)

Take a look at the man page for checkout for more information on --orphan.