Add messaging to phabricator (facebook#48470)

Summary:
Pull Request resolved: facebook#48470

This introduces:
- `validate_rn_cpp_api`: run a shell script that fails if the user changes the C++ API.  Importantly it produces an artifact with details to share with the user: `message`.
- `message` has to be extracted from `stdout`, which **get_user_message** does if the `validate_rn_cpp_api` action is not successful (**warning**).
- The then users `comment_to_signalhub` to share this as a warning to the user (until we're confident this entire stack is very stable, at which time we'll block).

The provides 2 classes of warning:
1. vanilla you've change the API,
2. you've change the API and haven't included the correct changelog.

Changelog: [Internal]

Reviewed By: GijsWeterings

Differential Revision: D67776215

fbshipit-source-id: 4ac7451c8ecef62ba968710ec41804ba42153976

Author: blakef

tools/api/check-api.sh

@@ -10,17 +10,20 @@ ERR_CODE_UNCOMMITTED_API_CHANGE=1
 ERR_CODE_CANT_FIND_PROJECT=2
 ERR_CODE_BAD_COMMIT=3
 
+# Delimit what we share with the users in PR / diffs
+MSG_START="=== MSG_START ==="
+MSG_END="=== MSG_END ==="
+
 WARNING_MESSAGE_HEADER=$(cat <<EOF
-🚨 React Native Public C++ / Objective-C / Objective-C++ API Change Detected:
+# React Native Public C++ / Objective-C / Objective-C++ API Change Detected:
 
 It looks like you've changed a public API that could affect our users, potentially breaking libraries and/or products using React Native.
 EOF
 )
 
 WARNING_MESSAGE_TAIL=$(cat <<EOF
 
-What to do now?
----------------
+### What to do now?
 
 Nothing for now, we're just tracking this. In the future we'll start adding folks from React Org as reviewers to check these changes align with our public API policy.
 
@@ -29,13 +32,12 @@ EOF
 
 ERROR_MENTION_BREAKING_CHANGE=$(cat <<EOF
 
-Commit Message:
----------------
+## Commit Message:
 
 If you did not intend to change React Native's Public API, please revert those changes in this commit. If you did intend to change the Public API, You *MUST* add a changelog entry to your commit message using the "Breaking" type (https://reactnative.dev/contributing/changelogs-in-pull-requests). For example:
 
-## Changelog:
-[General][Breaking] - Removed the Foo method to Bar as part of our deprecation mentioned in RFC-123
+   ## Changelog:
+   [General][Breaking] - Removed the Foo method to Bar as part of our deprecation mentioned in RFC-123
 
 EOF
 )
@@ -44,6 +46,7 @@ function check_if_mentions_breaking_change() {
     IN_COMMIT_MESSAGE=$(hg log -r . -T '{desc}' | grep -iE '\[(android|ios|general|internal)\]\s*\[breaking\]' || true)
     if [ -z "$IN_COMMIT_MESSAGE" ]; then
         echo "$ERROR_MENTION_BREAKING_CHANGE"
+        echo "$MSG_END"
         exit $ERR_CODE_BAD_COMMIT
     fi
 }
@@ -82,11 +85,23 @@ if [ -z "$API_STATUS" ]; then
     echo "🎉 No public API changes, happy days!"
     exit 0
 else
+    echo "$MSG_START"
+    echo
     echo "$WARNING_MESSAGE_HEADER"
+    echo
+    echo "### Overview:"
+    echo "\`\`\`"
+    hg diff --stat
+    echo "\`\`\`"
+    echo "### Differences:"
+    echo "\`\`\`diff"
+    hg diff "$API_FILE"
+    echo "\`\`\`"
 
     # Add any checks before showing the default warning
     check_if_mentions_breaking_change
 
     echo "$WARNING_MESSAGE_TAIL"
+    echo "$MSG_END"
     exit $ERR_CODE_UNCOMMITTED_API_CHANGE
 fi