Update HISTORY.md for PR6069 (#7663)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7663

Reviewed By: ajkr

Differential Revision: D24913081

Pulled By: riversand963

fbshipit-source-id: 704f427812f2b4f92e16d6cbc93be64d730d1cf9

HISTORY.md

@@ -7,7 +7,7 @@
 * Since 6.14, fix false positive flush/compaction `Status::Corruption` failure when `paranoid_file_checks == true` and range tombstones were written to the compaction output files.
 * Since 6.14, fix a bug that could cause a stalled write to crash with mixed of slowdown and no_slowdown writes (`WriteOptions.no_slowdown=true`).
 * Fixed a bug which causes hang in closing DB when refit level is set in opt build. It was because ContinueBackgroundWork() was called in assert statement which is a no op. It was introduced in 6.14.
-* Perform post-flush updates of in-memory data structures of memtable list in a callback that will be called by the thread writing to MANIFEST file before signaling other threads and releasing db mutex.
+* Fixed a bug which causes Get() to return incorrect result when a key's merge operand is applied twice. This can occur if the thread performing Get() runs concurrently with a background flush thread and another thread writing to the MANIFEST file (PR6069).
 * Reverted a behavior change silently introduced in 6.14.2, in which the effects of the `ignore_unknown_options` flag (used in option parsing/loading functions) changed.
 * Reverted a behavior change silently introduced in 6.14, in which options parsing/loading functions began returning `NotFound` instead of `InvalidArgument` for option names not available in the present version.
 * Fixed MultiGet bugs it doesn't return valid data with user defined timestamp.