Enable test execution behind firewall

This change is required to enable test execution behind
a proxy/firewall using parameters -DproxyHost=<host> -DproxyPort=<port>.

CliGitAPIImplTest.test_submodule_tags_not_fetched_into_parent() fails
also with proxy settings since git:// requires port 9418 to be open.
Using https:// solves this issue.

Mentioned test is the last failing test behind a firewall. If this is fixed all tests
can be properly executed behind a firewall using -DproxyHost=<host> -DproxyPort=<port>.