Add an (optional, since expensive) test for >2gb clones

Add an (optional, since expensive) test for >2gb clones

Define GIT_TEST_CLONE_2GB=t if you want the test not to be skipped.

The test works by constructing a repository larger than 2gb, and then
cloning it.

The repository is forced larger than 2gb by setting compression and
delta depth to zero, and then adding just enough unique objects of
a given size.

The objects consist of a running decimal number in ASCII, padded by
spaces.  Should that break in the future, e.g. when pack v4 becomes
default, there is a commented-out call to test-genrandom which can be
substituted, but that uses more cycles than the current method.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>