Fix JS API breaking change detection to compare against merge base

[emily8rown]

Summary

Updates the diff-js-api-breaking-changes action to compare the PR's changes against the merge base (where the branch diverged from main) instead of the current tip of main.

Problem

The previous implementation compared the current state of main to the PR head. This could produce false positive results when main had new commits to reactNativeApi.d.ts reporting breaking changes from main as if they were introduced by the PR.

Solution

• Calculate the merge base between the PR head and origin/main
• Compare the API snapshot at the merge base to the snapshot at the PR head

Changelog:

[GENERAL] [FIXED] - Updates the diff-js-api-breaking-changes action to compare the PR's changes against the merge base (where the branch diverged from main) instead of the current tip of main.

Test Plan:

(outputs shown for #54815)
Tested locally on branch that previously got false positive:
Mirror old approach:

mkdir -p /tmp/api-diff-old
git show origin/main:packages/react-native/ReactNativeApi.d.ts > /tmp/api-diff-old/before.d.ts
git show HEAD:packages/react-native/ReactNativeApi.d.ts > /tmp/api-diff-old/after.d.ts
node ./scripts/js-api/diff-api-snapshot /tmp/api-diff-old/before.d.ts /tmp/api-diff-old/after.d.ts

output:

{
  "result": "BREAKING",
  "changedApis": [
    "ActivityIndicatorProps",
    "Animated",
    "DrawerLayoutAndroidProps",
    "FlatList",
    "FlatListProps",
    "ImageBackground",
    "ImageBackgroundProps",
    "ImageProps",
    "ImagePropsBase",
    "KeyDownEvent",
    "KeyEvent",
    "KeyUpEvent",
    "KeyboardAvoidingView",
    "KeyboardAvoidingViewProps",
    "ModalProps",
    "PressableProps",
    "ProgressBarAndroidProps",
    "RefreshControl",
    "RefreshControlProps",
    "ScrollViewProps",
    "SectionList",
    "SectionListProps",
    "SwitchProps",
    "TextInputProps",
    "ViewProps",
    "VirtualizedListProps",
    "VirtualizedSectionListProps"
  ]
}

Mirror new approach:

git fetch origin main
MERGE_BASE=$(git merge-base HEAD origin/main)
echo "Merge base: $MERGE_BASE"
mkdir -p /tmp/api-diff
git show $MERGE_BASE:packages/react-native/ReactNativeApi.d.ts > /tmp/api-diff/before.d.ts
git show HEAD:packages/react-native/ReactNativeApi.d.ts > /tmp/api-diff/after.d.ts
node ./scripts/js-api/diff-api-snapshot /tmp/api-diff/before.d.ts /tmp/api-diff/after.d.ts 

output:

{
  "result": "NON_BREAKING",
  "changedApis": []
}

[meta-codesync]

@huntie has imported this pull request. If you are a Meta employee, you can view this in D88654826.

[meta-codesync]

@emily8rown merged this pull request in e7f5618.

[react-native-bot]

This pull request was successfully merged by @emily8rown in e7f5618

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