Make 2-phase nature of git opt-in.

The goal is to hide two-step commit as an implementation detail. It should still be possible but as an opt-in, not as a default. Additionally this is meant to be much easier to use. Heavily inspired by hg.

Roadmap:

• status - list files with pending changes
  • status --rev - show difference from revision
  • status --change - list the changed files of a revision
• commit - save all pending changes or specified files in a new commit
  • commit --amend
  • amend - shortcut for commit --amend
• diff - show differences between commits
  • diff --rev - show difference from revision
  • diff --change - list the changed files of a revision
  • diff --color - when to colorize (boolean, always, or nevers)
• record - interactively select changes to commit
• update - interactively select changes to commit
• smartlog - show a graph of the commits that are relevant to you