RevSpec parsing

[Byron]

Basic Parsing

It would be great to be part of the development in case you want to open a draft PR sooner than later. Parsing would go into the git-revision crate and have the following requirements.

• parse plain bytes (or &BStr), not &str
• call functions to signal which item was parsed, that is it doesn't parse into a data structure

Also see this discussion for more insights.

• a callback based parser

Revision Ranges

Prerequisistes

• Git Revision Parsing - See Revision Parsing #408
• Git Merge Base - See Mergebase support #409
• .. (two-dot) Range Notation
• ... (three-dot) Symmetric Difference Notation