git absorb

This is a port of Facebook's hg absorb. Facebook demoed hg absorb which is probably the coolest workflow enhancement I've seen to version control in years. Essentially, when your working directory has uncommitted changes on top of draft changesets, you can run hg absorb and the uncommitted modifications are automagically folded ("absorbed") into the appropriate draft ancestor changesets. This is essentially doing hg histedit + "roll" actions without having to make a commit or manually make history modification rules. The command essentially looks at the lines that were modified, finds a changeset modifying those lines, and amends that changeset to include your uncommitted changes. If the changes can't be made without conflicts, they remain uncommitted. This workflow is insanely useful for things like applying review feedback. You just make file changes, run hg absorb and the mapping of changes to commits sorts itself out. It is magical.