RFR: 2312: Do not require re-review for a simple merge [v6]

[Pavel Rappo]

> If a repository is configured to ignore stale reviews, every commit to a PR made against that repo means that the PR needs to be re-reviewed. Re-reviewing is costly, and not all commits are worth that cost.
> 
> One example of such a commit is a simple merge. During a PR's lifetime, the PR's target branch (typically, repo's `master`) might be merged into the PR multiple times. Usually, such merges are trivial and would effectively be performed by Skara automatically if the PR didn't have them in the first place [^*]. For such commits, it makes sense to not require review.
> 
> This is my first contribution to Skara domain logic, which I've just started learning about. Also, while I tried to code this change according to Skara customs and idioms, I might have missed something. I guess what I'm saying is **take extra care** reviewing this PR. Thanks.
> 
> 
> [^*]: This is the main assumption of this PR. Any simple merges present in a PR change the end result of that PR insignificantly, if at all.

Pavel Rappo has updated the pull request incrementally with four additional commits since the last revision:

 - Clean up
 - Cache targetHash as suggested
 - Cache Review coverage as suggested
 - Make PullRequest a ctor parameter as suggested

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1672/files
  - new: https://git.openjdk.org/skara/pull/1672/files/18ecf690..739c1191

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1672&range=05
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1672&range=04-05

  Stats: 39 lines in 5 files changed: 28 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1672.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1672/head:pull/1672

PR: https://git.openjdk.org/skara/pull/1672