This is to address #471 that
documented unhelpfull behavour when tracking git sources.

Add warning to git track if track and ref are not present

Closes #471

See merge request !580

This reverts commit 4f168b9b.

The latest version of pytest_pylint works fine with pylint 2, which
means there is no longer a reason to restrict the pylint version.

pylint 2 is required for Python 3.7.

Do not rely on `StopIteration` bubbling up.

https://www.python.org/dev/peps/pep-0479/

Python 3.7 support

See merge request !615

When the source's URL uses an alias (i.e. Source.translate_url()
actually changes the URL), we use the list of submodules defined in the
config as the list of submodules used.
This is because the source would otherwise have to fetch the main repo
to know which submodules it actually has.
This is inconvenient when using mirroring, because when the upstream
repo is unreachable it'll fail when it doesn't have to.

GitMirrors can now be instantiated without having their path and ref
defined, provided the 'parent' mirror is defined. It will retrieve the
path and ref as-needed instead.

If the source's URL does not use an alias, it will behave as normally.