Fix the race-condition fix, so it works in NRI CI

NoRedInk CI merges in master before running anything in CI. This means
that taking HEAD from git includes all master changes merged in.

This is what was causing the race condition fix in 0.13.3 to trigger
absurd checks for simple branches.

This commit introduces an option to override the head commit used in the
checks, so we can pass e.g. the GIT_COMMIT env var from Jenkins and make
the checks sane again.

Author: omnibs

exe/pr-checklist.rb

@@ -11,7 +11,7 @@
 
 # options and validation
 class Options
-  attr_writer :git_dir, :branch, :token, :org, :repo, :dry_run, :checklist
+  attr_writer :git_dir, :branch, :head_commit, :token, :org, :repo, :dry_run, :checklist
 
   # Use the supplied git dir, GIT_DIR from environment, find the .git directory
   # in a parent directory recursively or fail out by using the current
@@ -35,6 +35,11 @@ def branch
     b
   end
 
+  def head_commit
+    # Fall back to 'HEAD' unless specified
+    @head_commit || 'HEAD'
+  end
+
   def token
     t = @token || ENV['GITHUB_TOKEN']
     raise "--token or GITHUB_TOKEN must be set" unless t
@@ -72,6 +77,11 @@ def checklist
     "Which branch should we examine?"
   ) { |branch| options.branch = branch }
 
+  opts.on(
+    "-hc", "--head-commit HEAD", String,
+    "Use a custom head commit. Useful if you merge other stuff in before running this check."
+  ) { |head_commit| options.head_commit = head_commit }
+
   opts.on(
     "-t", "--token token", String,
     "Github access token (default: ENV['GITHUB_TOKEN'])"
@@ -120,7 +130,10 @@ def checklist
 # 3. We ask for `pr.head`, which is newer than `git.object('HEAD')`
 #
 # This race condition breaks commands that rely on `head` below
-head = git.object('HEAD').sha
+#
+# We also allow overriding the HEAD, since CI might change HEAD in a way that
+# interferes with checks here. (e.g. merging in master before running CI)
+head = git.object(options.head_commit).sha
 
 # Calculate the common ancestor where head diverged from pr.base, so the diff
 # is of the unique changes in head and not the changes that have since merged