Reports for PR runs should contain commit hashes

[mschwaig]

I often read the results of a nixpkgs-revew run from ~/.cache/nixpkgs-review/pr-[pr-number].

One of the files in there, probably the report itself, should contain hashes of the commits in question, so that it's clear, without having to rely on my own memory, what exact version of the package set was actually built.

The relevant commit hashes are:

• The tip of PR branch in question
• The tip of the target branch, if --checkout merge is used
• Maybe also the commit hash of the merge commit, if --checkout merge is used

[Mic92]

Sounds like a good idea.