Support git branch dependencies in Poetry / uv

[n-thumann]

Is there an existing issue for this?

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Feature description

This is a follow-up of #13584, as requested in #13584 (comment):

I noticed that Dependabot seems to be unable to handle git dependencies with branch ref for Poetry or uv, like

dependabot-core/python/spec/fixtures/pyproject_files/git_dependency.toml

Line 12 in 7448483

toml = { git = "https://github.com/uiri/toml.git", branch = "master" }

#13944 added support for tag-based git refs, while e.g. branches are still unsupported.