Will git have a baseline feature or something alike?

All,

I kept a mirror of

http://www.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

by a crontab task fetching the updated commits at midnight everyday.

Yet I found the repository now grows to be 1.2G without checking out
anything. The checked out working tree of this is about 1.5G.

I tried "git prune" and "git repack" but it still remains so large. The
trend of the kernel is still going to be enlarged. Thus I'm thinking
of the possibility of a baseline feature. One can totally forget about
the history before that baseline, and start the development there
after.

E.g.

1. user downloads a released tarball
2. and build a repository
3. and "git fetch" will find the current repository is identical to
a baseline in the remote, and fetches only commits after
that baseline
4. continue the development work

The above steps with current git will generate a totally different
hash value for the files in the downloaded tarball, thus making
it failed to fetch commits thereafter.

I know the history is usually mixed with multiple branches, which
makes this baseline feature a bit difficult to implement. It should
be a nice feature, though.

-- 
Cheers
- eric