Allow including the previous image tag in the commit message template

[darkowlzz]

Discussed in fluxcd/flux2#3027

^{Originally posted by tun0 August 24, 2022}
The notifications we currently have with Flux v1 (with help of fluxcloud) mention both old and new image tags.

With Flux v2, only the new image tag is mentioned.

It would be very nice if the previous tag would be made available as well in the notification object.

[darkowlzz]

Based on the discussion in fluxcd/image-reflector-controller#311 (comment) , this is not related to the image selected by the ImagePolicy, but it's about the old value being replaced by the new value in manifest file in git.
An ImageUpdateAutomation notification can include the information about the old and new value in the manifest that it updated.