Add --first-parent-baseline flag

[brady-zip]

Adds an opt-in --first-parent-baseline [branch] flag that passes --first-parent to the git rev-list ancestry walk in nextCommits, so baseline resolution stays on the mainline instead of descending into merged-in branch histories.

Context and motivation: #1449. This is the smaller of the two flags proposed there, and it carries the plumbing that the second one (--ignore-merged-pr-builds) builds on, so it goes first.

Behaviour

Default (flag unset) behaviour is byte-identical to today — the flag only ever adds --first-parent to the rev-list invocation.

Branch-glob scoped, following --ignore-last-build-on-branch exactly, so it can be enabled on the trunk only while PR builds keep the default base-branch baseline:

# .github/workflows/…
with:
  firstParentBaseline: master

// chromatic.config.json
{ "firstParentBaseline": "master" }

--first-parent-baseline with no value means true (via trueIfSet), matching --auto-accept-changes and friends.

Implementation

Standard option plumbing, one change per file:

File	Change
node-src/lib/parseArguments.ts	Help text under Chromatic options (alphabetical) and flags entry
node-src/types.ts	Flags.firstParentBaseline, Options.firstParentBaseline
node-src/lib/getOptions.ts	defaultOptions entry + trueIfSet(flags.firstParentBaseline)
node-src/lib/getConfiguration.ts	Zod z.union([z.string(), z.boolean()])
action.yml	firstParentBaseline input (alphabetical)
action-src/main.ts	getInput + runNode flags entry
node-src/tasks/gitInfo.ts	GitInfoInput + extractGitInfoInput, resolved via git.matchesBranch(firstParentBaseline || false)
node-src/git/getParentCommits.ts	Boolean threaded getParentCommits → step → nextCommits, interpolated into the rev-list command

The glob is resolved in the gitInfo task and getParentCommits receives a plain boolean, so node-src/git/ stays glob-unaware — same seam ignoreLastBuildOnBranch uses.

Testing

• node-src/git/getParentCommits.test.ts: new firstParentBaseline case on the simpleLoop fixture (F = merge(D, E), D is F's first parent) asserting both halves in one test — without the option E is included (it is a genuine ancestor), with the option the walk never reaches it. Because this suite runs getParentCommits against real generated git repositories, the assertion only holds if --first-parent actually reaches git rev-list.
• node-src/lib/getOptions.test.ts: flag → option for the glob form, and the bare-flag trueIfSet path; plus firstParentBaseline: undefined added to the defaults assertion.
• yarn typescript:check, yarn lint, yarn build, yarn test all clean locally (1007 passing).

Labels

Requesting minor + release — I don't have permission to set them.