Allow iterative rebase to select commits (contents) for edition

Currently, I have to go to the command line and do `git rebase -i HEAD~3`, then this appears:

pick f7f3f6d changed my name a bit
pick 310154e updated README formatting and added blame
pick a5f4a0d added cat-file

# Rebase 710f0f8..a5f4a0d onto 710f0f8
#
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#  x, exec = run command (the rest of the line) using shell
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

So, if I would like to edit some commit contents, I would change it to:

pick f7f3f6d changed my name a bit
pick 310154e updated README formatting and added blame
edit a5f4a0d added cat-file

Then, git would use commit, but stop for amending. Allowing me to freely edit the git history, amending changes the the picked commit, or adding new commits after it. Once I am done with it, I can do `git rebase --continue` and git would rewrite the history with the new updates.

The current SmartGit interface for Iterative Rebase only allows-me to edit the commit message, it cannot edit like `use commit, but stop for amending`.