feat(leanpkg): add git branch tracking for dependencies

[khoek]

Add branch tracking for git dependencies. There is no functional change to currently existing leanpkg.toml manifests. In a git dependency an optional branch="..." (along with the required git=... and rev=...) is now permitted, which will cause subsequent invocations of leanpkg upgrade to track that branch, as opposed to a default determined by the Lean version.

We opt to store the branch name inside an option string, instead of just picking a default the first time leanpkg.toml is read with it missing, since then when leanpkg.toml is re-written unexpected branch = "..." entries will not appear (as was the case with the old behaviour).

[Kha]

Looks great, thanks!