graph: resolve lineage in submodules

[jirikuncar]

(closes #120)

[rokroskar]

renga log just follows the symlink -- should it also state that the file comes from a submodule explicitly?

$ renga log woop.wc
*  3593c280 woop.wc
*  3593c280 .renga/workflow/5b2ea16447154a93bbbbf4ac16bb93c4_wc.cwl
@  73fd3ec1 .renga/vendors/local/Users/rok/temp/foo/data/dataset/woop

[rokroskar]

not sure what I should expect to see here or whether this is intended. I've got two repositories, one has imported a file from the other. I've changed the file in the submodule, I've done

git submodule --rebase --remote
git commit -a -m 'update submodule'

and then expected to see the submodule info in renga status but I get:

renga status
On branch master
Files generated from outdated inputs:
  (use "renga log <file>..." to see the full lineage)

	woop.wc:

[jirikuncar]

@rokroskar can you provide me full history of the commands you run to reproduce the problem? Normally it should display: <name_of_submodule>@<commit_sha1>. Please note that the history for datasets works only if the submodule is a Renga repository.