`bst workspace open` dislikes git sources that have submodules

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/757)
In GitLab by [[Gitlab user @coldtom]](https://gitlab.com/coldtom) on Nov 8, 2018, 10:41

## Summary

Attempting to run `bst workspace open` caused me a stack trace, I think due to the submodules in a git element, but this may not be the case.

## Steps to reproduce

1. Clone freedesktop-sdk
2. Run `bst workspace open desktop/gst-libav gst-libav`

## What is the current bug behavior?

The following stack trace:
```
[--:--:--][][] START Loading elements
[00:00:01][][] SUCCESS Loading elements
[--:--:--][][] START Resolving elements
[00:00:01][][] SUCCESS Resolving elements
[--:--:--][][] START Resolving cached state
[00:00:00][][] SUCCESS Resolving cached state
[--:--:--][][] START Checking sources
[00:00:00][][] SUCCESS Checking sources
[--:--:--][d4ca660b][ main:desktop/gstreamer-libav.bst ] START Staging sources to here
[--:--:--][ ][ main:desktop/gstreamer-libav.bst-0 ] STATUS Running host command

/usr/bin/git show 1.14.0-0-g1830c75cd28fc68bd26f9530db18c2da474a4aa2:.gitmodules

[--:--:--][ ][ main:desktop/gstreamer-libav.bst-0 ] STATUS Running host command

/usr/bin/git ls-tree 1.14.0-0-g1830c75cd28fc68bd26f9530db18c2da474a4aa2 common

[--:--:--][ ][ main:desktop/gstreamer-libav.bst-0 ] STATUS Running host command

/usr/bin/git ls-tree 1.14.0-0-g1830c75cd28fc68bd26f9530db18c2da474a4aa2 gst-libs/ext/libav

[--:--:--][ ][ main:desktop/gstreamer-libav.bst-0 ] STATUS Running host command

/usr/bin/git ls-tree 1.14.0-0-g1830c75cd28fc68bd26f9530db18c2da474a4aa2 gst-libs/ext/gas-preprocessor

[--:--:--][ ][ main:desktop/gstreamer-libav.bst-0 ] START Setting up workspace "/home/tom/.cache/buildstream/build/workspace-desktop-gstreamer-libavyrmfg9lh"
[--:--:--][][] BUG Primary URL marked twice with different URLs

Traceback (most recent call last):
File "/home/tom/.local/bin/bst", line 8, in
sys.exit(cli())
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File "/home/tom/Documents/work/buildstream/buildstream/_frontend/cli.py", line 162, in override_main
standalone_mode=standalone_mode, **extra)
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/home/tom/.local/lib/python3.7/site-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/home/tom/.local/lib/python3.7/site-packages/click/decorators.py", line 27, in new_func
return f(get_current_context().obj, *args, **kwargs)
File "/home/tom/Documents/work/buildstream/buildstream/_frontend/cli.py", line 705, in workspace_open
force=force)
File "/home/tom/Documents/work/buildstream/buildstream/_stream.py", line 513, in workspace_open
target._open_workspace()
File "/home/tom/Documents/work/buildstream/buildstream/element.py", line 1810, in _open_workspace
source._init_workspace(temp)
File "/home/tom/Documents/work/buildstream/buildstream/source.py", line 672, in _init_workspace
self.init_workspace(directory)
File "/home/tom/Documents/work/buildstream/buildstream/plugins/sources/git.py", line 408, in init_workspace
mirror.init_workspace(directory)
File "/home/tom/Documents/work/buildstream/buildstream/plugins/sources/git.py", line 220, in init_workspace
url = self.source.translate_url(self.url)
File "/home/tom/Documents/work/buildstream/buildstream/source.py", line 455, in translate_url
self.mark_download_url(url, primary=primary)
File "/home/tom/Documents/work/buildstream/buildstream/source.py", line 502, in mark_download_url
"Primary URL marked twice with different URLs"
AssertionError: Primary URL marked twice with different URLs
```

## What is the expected correct behavior?

Workspace should open correctly.

* BuildStream version affected: /milestone %BuildStream_v1.2

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