Support passing branches, commits, etc

[marinofelipe]

Currently the tool only support tags, and defaults to the latest tag, which doesn't work for all scenarios, e.g. when the latest tag doesn't contain a valid Package.swift in the repository root, as reported by @ipedro.

A concrete example is https://github.com/dzenbot/DZNEmptyDataSet.

The goal with this issue is to evaluate and add support for a more flexible approach, in which not only tags can be passed, but also branches, revision items, and where the behavior of the tool is a bit more predictable.

Some top of mind ideas for that would be to either:

• Additional Optional argument(s), like branch and revision
• Or update -v to be a wild card argument, that can be named either v, branch or revision, that allows passing any of those and smartly resolves it

[marinofelipe]

After #44 this got simpler, the SwiftPM lib has built in support for checking out a revision

[marinofelipe]

Shipped with the 1.5.0 release