skip some git submodule failures

git submodule add with a non-existing path fails
for git between 1.7.0.rc1 and 1.7.0.2
remove a useless test that fails with git < 1.5.1.rc1

git --pretty:format=%s is not supported until git 1.5.1.rc1
remove the now useless output test
skip some more git submodule failures

git submodule add with a non-existing path fails
for all subversions of git 1.5.4 and git 1.5.5
git init is actually available since git 1.5.0.rc1
also test with the release candidates for git 1.5.0 when possible

test_repository (from Test::Git) actually works with at least git 1.5.0.rc1

when that function is not used, move the required version
as far as 1.5.0.rc0 when possible