Simplified rebase
Simplified rebase

- Removed stateful `SubmoduleRebaser` class.  It had several methods
  that needed to be invoked at the right time.  This design was
  overly-compex and unnecessary.  Now, we invoke one method for
  performing a single submodule rebase.
- Made rebase operations serial for now.  I believe this change should
  address most outstanding issues with rebase, but want to rule out
  parallelism as a potential source of instability.
- Moved the logic to deal with conflicted `.gitmodules` file into a
  separate method and removed the unnecessary laziness.
- moved per-operation logic into `processMetaRepoRebaseOp`
- this eliminated to complex statefulness where several variables had to
  be reassigned after each operation
- moved per-index-entry logic into `processMetaRebaseEntry`

twosigma/git-meta,jhedwardyang/git-meta