How to inject a commit between some two arbitrary commits in the past?

Suppose I have the following commit history on my local-only branch:

A -- B -- C

How do I insert a new commit between A and B?

It's even easier than in OP's answer.

git rebase -i <any earlier commit>. This displays a list of commits in your configured text editor.
Find the commit you want to insert after (let's assume it's a1b2c3d). In your editor, for that line, change pick to edit.
Begin the rebase by closing your text editor (save your changes). This leaves you at a command prompt with the commit you chose earlier (a1b2c3d) as if it has just been committed.
Make your changes and git commit (NOT amending, unlike most edits). This creates new a commit after the one you chose.
git rebase --continue. This replays the successive commits, leaving your new commit inserted in the correct place.

Beware that this will rewrite history, and break anyone else who tries to pull.

Turns out to be quite simple, the answer found here. Suppose you're on a branch branch. Perform these steps:

create a temporary branch from the commit after you want to insert the new commit (in this case commit A):
```
git checkout -b temp A
```
perform the changes and commit them, creating a the commit, let's call it N:
```
git commit -a -m "Message"
```
(or git add followed by git commit)
rebase the commits you want to have after the new commit (in this case commits B and C) onto the new commit:
```
git rebase temp branch
```

^{(possibly you need to use -p to preserve merges, if there were any - thanks to a no longer existing comment by ciekawy)}

After this, the history looks as follows:

A -- N -- B -- C

It is of course possible that some conflicts will appear while rebasing.

In case your branch is not local-only this will introduce rewriting history, so might cause serious problems.

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