Improve hg revset used by jest-changed-files

The old revset is `ancestor(.^, min(branch(.) and not min(branch(default)), max(public()))`.
It can select public commits unintentionally in some cases.

In the following two examples, commit A would be selected by the old code path
as the "from revision". The new revset would select commit C instead, which
is better because the developer won't want to test commits in the `B::C` range.

Example 1 - Multiple named branches:

    E default (named branch), public
    :
    | D stable (named branch), draft (only commit the developer cases about)
    | |
    | C stable (named branch), public
    | :
    | |
    | B stable (named branch), public
    |/
    A

Example 2 - Multiple heads in the "default" named branch:

    E default (named branch), public, has a bigger revision number than C.
    :
    | D default (named branch), draft (only commit the developer cases about)
    | |
    | C default (named branch), public
    | :
    | |
    | B default (named branch), public
    |/
    A


Explanation of the `min(!public() & ::.)^` revset:

With modern Mercurial, `!public()` is the way to select local commits that
do not exist on other developer's repos. `& ::.` limits commits to only the
current "feature branch" (branch in terms of the commit DAG, not hg named
branches).  `min` selects the first commit in the non-public feature branch,
and `^` selects its immediate parent. Note: it's `!public() & ::.` instead
of `::. & !public()` intentionally, because the former has a fast path [1].

[1]: See https://www.mercurial-scm.org/repo/hg/rev/c6c8a52e28c9077580dd2f0552eb2bd6d5e0d13c

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixes
 
 - `[jest-cli]` Refactor `-o` and `--coverage` combined ([#7611](https://github.com/facebook/jest/pull/7611))
+- `[jest-changed-files]` Improve default file selection for Mercurial repos ([#7880](https://github.com/facebook/jest/pull/7880))
 
 ### Chore & Maintenance
 

packages/jest-changed-files/src/hg.ts

@@ -13,17 +13,6 @@ import {Path, Options, SCMAdapter} from './types';
 
 const env = {...process.env, HGPLAIN: '1'};
 
-const ANCESTORS = [
-  // Parent commit to this one.
-  '.^',
-
-  // The first commit of my branch, only if we are not on the default branch.
-  'min(branch(.)) and not min(branch(default))',
-
-  // Latest public commit.
-  'max(public())',
-];
-
 const adapter: SCMAdapter = {
   findChangedFiles: async (
     cwd: string,
@@ -33,7 +22,7 @@ const adapter: SCMAdapter = {
 
     const args = ['status', '-amnu'];
     if (options && options.withAncestor) {
-      args.push('--rev', `ancestor(${ANCESTORS.join(', ')})`);
+      args.push('--rev', `min(!public() & ::.)^`);
     } else if (options && options.changedSince) {
       args.push('--rev', `ancestor(., ${options.changedSince})`);
     } else if (options && options.lastCommit === true) {