For server-side detection of rebases and commit amends, we need to keep

track of some metadata indicating the original SHA1 of commits. With

git, we can store this metadata in git notes.
git notes allows for storing arbitrary metadata associated with each

commit. We store the SHA1 of the commit in a special notes ref,

rbt-history-metadata. This way, we do not interfere with notes that

users may be using. We also add this ref to the list of refs to be

rewritten during rebases and amends.
When an amend occurs, the SHA1 of the commit will change, but the

stored SHA1 will not, so we will be able to detect that commit was

amended. Likewise, when a squash occurs during a rebase, the SHA1 will

change but the stored metadata will be the SHA1 hashes of all the

squashed commits, so we can also know when a squash occurs. We can then

upload this metadata when doing rbt post so that Review Board can

detect history changes server side.

Using https://reviews.reviewboard.org/r/8276/, I was able to do the

following:

Upload a review request with multiple commits.
Squash the commits together locally.
Update the review request.

I observed that the original commits were set in the Review Board

database.