Find size of Git repository

What's a simple way to find the size of my Git repository?

And I don't mean du -h on the root directory of my repository. I have a lot of ignored files, so that size would be different from my total repository size. I essentially want to know how much data would be transferred upon cloning my repository.

UPDATE git 1.8.3 introduced a more efficient way to get a rough size: git count-objects -vH (see answer by @VonC)

For different ideas of "complete size" you could use:

git bundle create tmp.bundle --all
du -sh tmp.bundle

Close (but not exact:)

git gc
du -sh .git/

With the latter, you would also be counting:

• hooks
• config (remotes, push branches, settings (whitespace, merge, aliases, user details etc.)
• stashes (see Can I fetch a stash from a remote repo into a local branch? also)
• rerere cache (which can get considerable)
• reflogs
• backups (from filter-branch, e.g.) and various other things (intermediate state from rebase, bisect etc.)