t0001: validate Git -rcN version numbers correctly

When creating the GIT-VERSION-FILE that we use to test that the version
of Git in git/ is the same as in the CGit Makefile, Git applies the
transform "s/-/./g" to the version string.  This doesn't affect released
versions but does change RC version numbers such as 1.8.3-rc0.

While CGit should only refer to a released Git version in general, it is
useful to developers who want to test upcoming Git releases if the tests
do work with RCs, so change t0001 to apply the same transform to our
Makefile version before comparing it to the contents of
GIT-VERSION-FILE.

Signed-off-by: John Keeping <john@keeping.me.uk>
git: update to 1.8.3-rc0

No changes required, just bump the submodule and Makefile versions.

Signed-off-by: John Keeping <john@keeping.me.uk>