Unify redate behaviour during cherry-picks

[cburkert]

Newer versions of Git (since when?) now trigger post-commit hooks during a cherry-pick which invokes redate. This causes a different user experience than for older version where no redating took place during cherry-picks.

[cburkert]

As of now, I found no reliable why to detect, whether post-commit is triggered as a result of a cherry-pick. Looking for the presence of CHERRY_PICK_HEAD is not reliable since it constitutes a race condition, because it vanishes as soon as the cherry pick is finished, which can be before, during or after the post-commit hook is invoked.

[cburkert]

Related to fc0047b

[fapdash]

@cburkert Can you detect a cherry pick at the time a pre-commit hook is triggered?

[cburkert]

@fapdash Strangely, my experiments show that a cherry-pick does not trigger a pre-commit hook. More indication that the triggering of post-commit is a bug of Git...