Merge #18283: doc: Explain rebase policy in CONTRIBUTING.md

fa12447 doc: Explain rebase/squash policy in CONTRIBUTING.md (MarcoFalke)

Pull request description:

ACKs for top commit:
  hebasto:
    re-ACK fa12447, typos fixed.
  laanwj:
    ACK fa12447

Tree-SHA512: b8f790a8ffa4f59f26b64befbea8acd92f3c548e577c35750a60dde94b4340258a540ad0db60a6de650db0156cbff9e1330c589c300db5253531fe6a16cdbcc9

Author: laanwj

CONTRIBUTING.md

@@ -117,12 +117,12 @@ At this stage, one should expect comments and review from other contributors. Yo
 can add more commits to your pull request by committing them locally and pushing
 to your fork until you have satisfied all feedback.
 
-Note: Code review is a burdensome but important part of the development process, and as such, certain types of pull requests are rejected. In general, if the **improvements** do not warrant the **review effort** required, the PR has a high chance of being rejected. It is up to the PR author to convince the reviewers that the changes warrant the review effort, and if reviewers are "Concept NAK'ing" the PR, the author may need to present arguments and/or do research backing their suggested changes.
+Note: Code review is a burdensome but important part of the development process, and as such, certain types of pull requests are rejected. In general, if the **improvements** do not warrant the **review effort** required, the PR has a high chance of being rejected. It is up to the PR author to convince the reviewers that the changes warrant the review effort, and if reviewers are "Concept NACK'ing" the PR, the author may need to present arguments and/or do research backing their suggested changes.
 
-Squashing Commits
----------------------------
-If your pull request is accepted for merging, you may be asked by a maintainer
-to squash and or [rebase](https://git-scm.com/docs/git-rebase) your commits
+### Squashing Commits
+
+If your pull request contains fixup commits (commits that change the same line of code repeatedly) or too fine-grained
+commits, you may be asked to [squash](https://git-scm.com/docs/git-rebase#_interactive_mode) your commits
 before it will be merged. The basic squashing workflow is shown below.
 
     git checkout your_branch_name
@@ -149,6 +149,20 @@ the respective change set.
 The length of time required for peer review is unpredictable and will vary from
 pull request to pull request.
 
+### Rebasing Changes
+
+When a pull request conflicts with the target branch, you may be asked to rebase it on top of the current target branch.
+The `git rebase` command will take care of rebuilding your commits on top of the new base.
+
+This project aims to have a clean git history, where code changes are only made in non-merge commits. This simplifies
+auditability because merge commits can be assumed to not contain arbitrary code changes. Merge commits should be signed,
+and the resulting git tree hash must be deterministic and reproducible. The script in
+[/contrib/verify-commits](/contrib/verify-commits) checks that.
+
+After a rebase, reviewers are encouraged to sign off on the force push. This should be relatively straightforward with
+the `git range-diff` tool explained in the [productivity
+notes](/doc/productivity.md#diff-the-diffs-with-git-range-diff). To avoid needless review churn, maintainers will
+generally merge pull requests that received the most review attention first.
 
 Pull Request Philosophy
 -----------------------