Support `branch` in git dependencies

[LesterEvSe]

Describe the feature

Simplex currently accepts tag and rev to pin a git dependency:

[dependencies]
std = { git = "https://github.com/BlockstreamResearch/simplicityhl-std.git", tag = "v0.1.0" } 
smth = { git = "<url>", rev = "<commit-sha>" }

There is no way to track a branch, so we want to support the following shape:
alias = { git = "<url>", branch = "<branch-name>" }

For example:

[dependencies]
std = { git = "https://github.com/BlockstreamResearch/simplicityhl-std.git", branch = "dev" }