Changes:

• Rewrite in Rust (single binary)
• Drop "prepare-metadata" script
• "push" and "fetch" now only support dummy auth and ssh-agent
• Updated README
• Updated man page

Added features:

• Documentation of data model, semantics and use-cases
• Library crate "libgitdit"
• Long options

Bug-fixes:

• Read editor from git config, fall back to default
  (We previously relied on the EDITOR variable.)