Fully backup a git repo?

git bundle

I like that method, as it results in only one file, easier to copy around.
See ProGit: little bundle of joy.
See also "How can I email someone a git repository?", where the command

git bundle create /tmp/foo-all --all

is detailed:

git bundle will only package references that are shown by git show-ref: this includes heads, tags, and remote heads.
It is very important that the basis used be held by the destination.
It is okay to err on the side of caution, causing the bundle file to contain objects already in the destination, as these are ignored when unpacking at the destination.

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For using that bundle, you can clone it, specifying a non-existent folder (outside of any git repo):

git clone /tmp/foo-all newFolder

Whats about just make a clone of it?

git clone --mirror other/repo.git

Every repository is a backup of its remote.