reposet: fetch remotes for externals
When repos are cloned into externals, they are cloned from {build_root}/repos/{project}, which is not bare.
To ensure that new refs are fetched, we need a ref spec that copies refs from the parent project.
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When repos are cloned into externals, they are cloned from {build_root}/repos/{project}, which is not bare.
To ensure that new refs are fetched, we need a ref spec that copies refs from the parent project.