In interactive mode, you can mark commits with the action "edit".
However, this does not necessarily mean that git rebase expects
the result of this edit to be exactly one commit. Indeed, you can
undo the commit, or you can add other commits. This can be used
to split a commit into two:
• Start an interactive rebase with git rebase -i <commit>^,
where <commit> is the commit you want to split. In fact, any
commit range will do, as long as it contains that commit.
• Mark the commit you want to split with the action "edit".
• When it comes to editing that commit, execute git reset
HEAD^. The effect is that the HEAD is rewound by one, and the
index follows suit. However, the working tree stays the same.
• Now add the changes to the index that you want to have in the
first commit. You can use git add (possibly interactively) or
git gui (or both) to do that.
• Commit the now-current index with whatever commit message is
appropriate now.
• Repeat the last two steps until your working tree is clean.
• Continue the rebase with git rebase --continue.
If you are not absolutely sure that the intermediate revisions
are consistent (they compile, pass the testsuite, etc.) you
should use git stash to stash away the not-yet-committed changes
after each commit, test, and amend the commit if fixes are
necessary.
