Store the minimum blob size in git's config file to use the same value across backups

If we use a different size across backup, it will not corrupt the data, but
it will generate a lot of new blobs because the data would be splitted
differently, thus creating bigger backups than necessary.

With a real-life files collection, the repositories size goes like this:

		Min size = 0	  Min size = 56		Diff
First commit	187,117,254,339   187,117,254,339	15,538,839

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Add some tests for the git's config methods