Add script for identifying commits in release branches

One of the worst parts of a Podman release is writing the release
notes. It requires manually going through all merged commits
since the last release, figuring out what was actually done, and
writing a small blurb about what was fixed. The worst part of
this is the difficulty in finding the commits that were actually
included in previous releases - our extensive backports to prior
releases mean that there are usually dozens of commits that were
included in a prior release, but do not have a matching SHA (as
the original author did not do the backport, and often the commit
required massaging to cherry-pick in).

This script automates the job of finding commits in one release
branch that are not in another, with filtering to remove most
cherry-picked commits. It makes my life a lot easier during
releases, so I figured I'd include it in hack/ so anyone else
stuck with the enjoyable task of writing release notes can have a
slightly easier life.

The script is written in absolutely terrible Ruby and its
performance is absolutely terrible, but you only need to run it
once per major release and a 30-second wait to generate the list
of commits to include isn't bad.

Signed-off-by: Matthew Heon <mheon@redhat.com>

hack/branch_commits.rb

@@ -0,0 +1,98 @@
+#!/usr/bin/ruby
+
+require 'set'
+
+# Get commits in one branch, but not in another, accounting for cherry-picks.
+# Accepts two arguments: base branch and old branch. Commits in base branch that
+# are not in old branch will be reported.
+
+# Preface: I know exactly enough ruby to be dangerous with it.
+# For anyone reading this who is actually skilled at writing Ruby, I can only
+# say I'm very, very sorry.
+
+# Utility functions:
+
+# Check if a given Git branch exists
+def CheckBranchExists(branch)
+  return `git branch --list #{branch}`.rstrip.empty?
+end
+
+# Returns author (email) and commit subject for the given hash
+def GetCommitInfo(hash)
+  info = `git log -n 1 --format='%ae%n%s' #{hash}`.split("\n")
+  if info.length != 2
+    puts("Badly-formatted commit with hash #{hash}")
+    exit(127)
+  end
+  return info[0], info[1]
+end
+
+# Actual script begins here
+
+if ARGV.length != 2
+  puts("Must provide exactly 2 arguments, base branch and old branch")
+  exit(127)
+end
+
+# Both branches must exist
+ARGV.each do |branch|
+  if !CheckBranchExists(branch)
+    puts("Branch #{branch} does not exist")
+    exit(127)
+  end
+end
+
+base = ARGV[0]
+old = ARGV[1]
+
+# Get a base list of commits
+commits = `git log --no-merges --format=%H #{base} ^#{old}`.split("\n")
+
+# Alright, now for the hacky bit.
+# We want to remove every commit with a shortlog precisely matching something in
+# the old branch. This is an effort to catch cherry-picks, where commit ID has
+# almost certainly changed because the committer is different (and possibly
+# conflicts needed to be resolved).
+# We will match also try and match author, but not committer (which is reset to
+# whoever did the cherry-pick). We will *not* match full commit body - I
+# routinely edit these when I fix cherry-pick conflicts to indicate that I made
+# changes. A more ambitious future committer could attempt to see if the body of
+# the commit message in the old branch is a subset of the full commit message
+# from the base branch, but there are potential performance implications in that
+# due to the size of the string comparison that would be needed.
+# This will not catch commits where the shortlog is deliberately altered as part
+# of the cherry pick... But we can just ask folks not to do that, I guess?
+# (A classic example of something this wouldn't catch: cherry-picking a commit
+# to a branch and then prepending the branch name to the commit subject. I see
+# this a lot in Github PR subjects, but fortunately not much at all in actual
+# commit subjects).
+
+# Begin by fetching commit author + subject for each commit in old branch.
+# Map each author to an array of potential commit subjects.
+oldIndex = {}
+
+# TODO: This could probably be made a whole lot more efficient by unifying the
+# GetCommitInfo bits into two big `git log --format` calls.
+# But I'm not really ambitious enough to do that...
+oldCommits = `git log --no-merges --format=%H #{old}`.split("\n")
+oldCommits.each do |hash|
+  name, subject = GetCommitInfo(hash)
+  if oldIndex[name] == nil
+    oldIndex[name] = Set[]
+  end
+  oldIndex[name].add(subject)
+end
+
+# Go through our earlier commits list and check for matches.
+filtered = commits.reject do |hash|
+  name, subject = GetCommitInfo(hash)
+  oldIndex[name] != nil && oldIndex[name].include?(subject)
+end
+
+# We have now filtered out all commits we want to filter.
+# Now we just have to print all remaining commits.
+# This breaks the default pager, but we can just pipe to less.
+filtered.each do |hash|
+  puts `git log -n 1 #{hash}`
+  puts "\n"
+end