Add changelog.md file.

Author: jefgen

changelog.md

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+# Changelog
+
+## ICU 64.2.0.1
+
+Changes/modifications compared to the upstream `maint/maint-64` branch.
+
+#### General changes:
+- Microsoft internal data changes.
+- Additional locales from CLDR to improve parity with existing Windows NLS locale support.
+- Update timezone data files to tz2019a
+  - Uses files from https://github.com/unicode-org/icu-data/tree/master/tzdata/icunew/2019a/44.
+
+#### Windows OS build-related:
+- Changes needed for the Windows OS build of ICU.
+- Make u_cleanup a no-op for Windows OS ICU
+- Conditionally set data file name based on ICU_DATA_DIR_WINDOWS to support Windows OS build with only a single data file.
+- Don't use the extended ICU data package for Windows OS build.
+- Add macro (FORCE_DISABLE_UMUTEX_CONSTEXPR) to control toggling the constexpr of the UMutex class.
+
+#### Changes cherry-picked from upstream tickets/PRs:
+
+Build Clean in VS doesn't fully clean everything that it should
+- https://unicode-org.atlassian.net/browse/ICU-13456
+- https://github.com/unicode-org/icu/commit/71ee05ed17e83d62f097467632c228e2447b5fde
+
+UInitOnce, make sure all instances are initialized.
+- https://unicode-org.atlassian.net/browse/ICU-20570
+- https://github.com/unicode-org/icu/commit/a97cfb01b917125fdb9705cf79786188cebb6e2e
+
+Locale::getKeywords() and Locale::getUnicodeKeywords() segfault on empty
+- https://unicode-org.atlassian.net/browse/ICU-20573
+- https://github.com/unicode-org/icu/commit/711e7e003a4e0a38d58baa07e64ecb52e9a71776
+
+ICU headers cannot be built within extern "C" scope
+- https://unicode-org.atlassian.net/browse/ICU-20530
+- https://github.com/unicode-org/icu/commit/0aa19c0d22af72a9b708b6285bbb1deb6cebcb62
+
+UMutex, static construction & destruction
+- https://unicode-org.atlassian.net/browse/ICU-20520
+- https://github.com/unicode-org/icu/commit/b772241b52fcec07b1909d44d156fbcd0cbfb20c
+
+Need to make TimeZone::AdoptDefault thread safe
+- https://unicode-org.atlassian.net/browse/ICU-20595
+- https://github.com/unicode-org/icu/commit/cb40d8b1a5eb6dd01e3972db5e1a05b672202f80
+
+ICU4C: Guard C++ public headers with C_SHOW_CPLUSPLUS_API
+- https://unicode-org.atlassian.net/browse/ICU-20578
+- https://github.com/unicode-org/icu/commit/a5bbd505d7a5f350409b42ebe075e24a4f2c63ce
+
+Missing </ClCompile> closing tag in the file "intltest.vcxproj.filters"
+- https://unicode-org.atlassian.net/browse/ICU-20613
+- https://github.com/unicode-org/icu/commit/b7ffd7b6d0d5fa7b8fbd850362b7165a89c6f3c2
+
+u_cleanup() should close OS level mutexes.
+- https://unicode-org.atlassian.net/browse/ICU-20588
+- https://github.com/unicode-org/icu/commit/afa9b9b48e0a50f2298e5d47c101ac3325629d8c
+
+DateTimePatternGenerator test fails when running with Valgrind
+- https://unicode-org.atlassian.net/browse/ICU-20625
+- https://github.com/unicode-org/icu/commit/f9ea5351b0a308b57ab951eea6b1136134a610fd
+
+Top-level .gitignore contains incorrect entries
+- https://unicode-org.atlassian.net/browse/ICU-20527
+- https://github.com/unicode-org/icu/commit/5b4befd67c2b99bc805091b6efa120c3fdd50268
+
+RegexCompile::compile crash with (?<![?&&?]?)
+- https://unicode-org.atlassian.net/browse/ICU-20544
+- https://github.com/unicode-org/icu/commit/7053363323ca3bf2a8853058f20c5e1c6b7e5024
+- https://github.com/unicode-org/icu/commit/bdb1806580702f05d7bac627f367c12d45183796
+
+Regex Failures with nested look-around expressions
+- https://unicode-org.atlassian.net/browse/ICU-20391
+- https://github.com/unicode-org/icu/commit/d685cacd9b020569bfad61869cea87a7d45bda87
+
+Fix Windows build failures with long paths: Use PowerShell when command length exceeds CMD's limit.
+- https://unicode-org.atlassian.net/browse/ICU-20555
+- https://github.com/unicode-org/icu/commit/c28505caaaefe6848be7fc760da04da2383a4f57
+
+Fix typo in API doc comments about unumf_openForSkeletonAndLocale.
+- https://unicode-org.atlassian.net/browse/ICU-20721
+- https://github.com/unicode-org/icu/pull/713
+
+UPRV_UNREACHABLE called in code that is easily reachable, crashes production code
+- https://unicode-org.atlassian.net/browse/ICU-20680
+- https://github.com/unicode-org/icu/pull/780
+- https://github.com/unicode-org/icu/pull/787
+
+Fix typo in API doc comments in ucurr.h
+- https://unicode-org.atlassian.net/browse/ICU-20794
+- https://github.com/unicode-org/icu/pull/785
+
+Fix memory leak in SimpleDateFormat::adoptCalendar, delete calendarToAdopt upon error
+- https://unicode-org.atlassian.net/browse/ICU-20799
+- https://github.com/unicode-org/icu/pull/790
+
+OOM not handled in NumberFormatterImpl::macrosToMicroGenerator
+- https://unicode-org.atlassian.net/browse/ICU-20368
+- https://github.com/unicode-org/icu/pull/795
+- https://github.com/unicode-org/icu/commit/543495da740441b4c89e585d55c8eb532b4cc595
+
+OOM not handled in uloc_openKeywordList, need to use LocalMemory
+- https://unicode-org.atlassian.net/browse/ICU-20802
+- https://github.com/unicode-org/icu/pull/796
+- https://github.com/unicode-org/icu/pull/805
+
+OOM not handled in selectForMask
+- https://unicode-org.atlassian.net/browse/ICU-20804
+- https://github.com/unicode-org/icu/pull/811
+
+Disable optimization for MSVC on ARM64 on versions below 16.4 to fix crashes
+- https://unicode-org.atlassian.net/browse/ICU-20907
+- https://github.com/unicode-org/icu/pull/926
+
+ICU-20845 UMutex not trivially but constexpr constructible #870 
+- https://unicode-org.atlassian.net/browse/ICU-20845
+- https://github.com/unicode-org/icu/pull/870
+- https://github.com/unicode-org/icu/commit/e5381c956b73b236c5d7866b2a6b65f879e770ea
+
+ICU-20958 Prevent SEGV_MAPERR in UnicodeString::doAppend()
+- https://unicode-org.atlassian.net/browse/ICU-20958
+- https://github.com/unicode-org/icu/pull/971
+- https://github.com/unicode-org/icu/commit/b7d08bc04a4296982fcef8b6b8a354a9e4e7afca
+
+Warn on global/static constructors
+- https://unicode-org.atlassian.net/browse/ICU-20493
+- https://github.com/unicode-org/icu/pull/708
+- https://github.com/unicode-org/icu/commit/fc487bf32b880298890161446e26d1077b672f74
+
+Initialization/deinitialization order of UMutex global statics is not defined with VS2017
+- https://unicode-org.atlassian.net/browse/ICU-21075 
+- https://github.com/unicode-org/icu/pull/1110
+