Fix overlay bug with multiple field patches

[wu-hui]

Fix: #3621

[google-oss-bot]

Coverage Report ¹

Affected Products

• firebase-firestore

  Overall coverage changed from 46.24% (1f25801) to 46.12% (0a13476) by -0.12%.

  14 individual files with coverage change

  Filename	Base (1f25801)	Merge (0a13476)	Diff
  AsyncQueue.java	78.61%	78.11%	-0.50%
  Datastore.java	35.53%	32.89%	-2.63%
  DeleteMutation.java	90.00%	90.48%	+0.48%
  FirestoreChannel.java	17.95%	16.24%	-1.71%
  FirestoreClient.java	38.52%	31.11%	-7.41%
  GrpcCallProvider.java	69.41%	62.35%	-7.06%
  IndexBackfiller.java	92.19%	76.56%	-15.63%
  LocalStore.java	99.37%	99.38%	+0.00%
  LruGarbageCollector.java	93.46%	84.11%	-9.35%
  Mutation.java	100.00%	98.48%	-1.52%
  MutationBatch.java	89.39%	89.06%	-0.33%
  OverlayedDocument.java	?	100.00%	?
  SetMutation.java	94.29%	97.22%	+2.94%
  VerifyMutation.java	50.00%	46.15%	-3.85%

Test Logs

• Base (1f25801): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1512488507900170240
• Merge (0a13476): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3623/check-coverage-changed/1513546503455510531

Notes

• Commit (0a13476) is created by Prow via merging PR base commit (1f25801) and head commit (f8d79c2).
• Run gradle <product>:checkCoverage to produce HTML coverage reports locally. After gradle commands finished, report files can be found under <product-build-dir>/reports/jacoco/.

1. https://storage.googleapis.com/firebase-sdk-metric-reports/YPFR7uMrT7.html

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-firestore

  Type	Base (1f25801)	Merge (0a13476)	Diff
  aar	1.25 MB	1.25 MB	+1.29 kB (+0.1%)
  apk (release)	3.38 MB	3.38 MB	+504 B (+0.0%)

Test Logs

• Base (1f25801): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1512488507900170241
• Merge (0a13476): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3623/binary-size/1513546503455510530

Notes

• Commit (0a13476) is created by Prow via merging PR base commit (1f25801) and head commit (f8d79c2).

1. https://storage.googleapis.com/firebase-sdk-metric-reports/E5YOf9jMAE.html

[ehsannas]

Also need a CHANGELOG entry?

[ehsannas]

LGTM, some nits below

[google-oss-bot]

@wu-hui: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
smoke-tests	f8d79c2	link	/test smoke-tests

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