BuildStream 1.4 not handling relative git URL's properly in submodules

Summary

BuildStream 1.4 not handling relative git URL's properly in submodules. URL's starting with ../ or ./ are supposed to be calculated from main Git URL.

Steps to reproduce

Have element that uses Git source with a submodule with relative URL.

What is the current bug behavior?

BuildStream tries to do a local clone.

What is the expected correct behavior?

BuildStream should clone a remote repo.

Relevant logs and/or screenshots

Possible fixes

We could calculate real URL at https://gitlab.com/BuildStream/buildstream/blob/1.4.1/buildstream/plugins/sources/git.py#L590 before creating GitMirror

Other relevant information

At least 1.4.1, also been present throughout 1.2.x. Not known if affects master.

• BuildStream version affected: /milestone %BuildStream_v1.x

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