t0001: ignore ".dirty" suffix on Git version

When testing modifications in Git that affect CGit, it is annoying to
have t0001 failing simply because the Git version has a ".dirty" suffix
when the version of Git there does indeed match that specified in the
CGit makefile.  Stop this by stripping the ".dirty" suffix from the
GIT_VERSION variable.

Note that this brings the "Git version" behaviour in line with the
"submodule version" case which does not check if the working tree in
git/ is modified.

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