* Update spotbugs-maven-plugin to 4.2.0

* Add org.eclipse.jetty.util.ajax to target platform and bazel deps

* Tag message must not include the signature

* Update jetty to 9.4.35.v20201120

* Protocol V2: don't log spurious ACKs in UploadPack

* Protocol V2: respect MAX_HAVES only once we got at least one ACK

* FileSnapshot: don't try to read file attributes twice

* RepositoryCache: declare schedulerLock final

* Enable git wire protocol version 2 on server side per default

* Client-side protocol V2 support for fetching

* Use Map interface instead of ConcurrentHashMap class

* Fix NPE in DirCacheCheckout

* Added check for null on DirCacheEntry in checkoutEntry method

* GPG user ID matching: use case-insensitive matching

* Don't export package from test bundle

* Update eclipse-jarsigner-plugin to 1.1.7
* Update build-helper-maven-plugin to 3.2.0
* Update maven-enforcer-plugin to 3.0.0-M3
* Update maven-shade-plugin to 3.2.4
* Require latest Maven version 3.6.3
* Update spring-boot-maven-plugin to 2.4.1
* Update maven-surefire plugins to 3.0.0-M5
* Update japicmp-maven-plugin to 0.14.4
* Update spotbugs-maven-plugin to 4.1.4
* Upgrade ecj to 3.24.0
* Upgrade wagon-ssh to 3.4.2; align maven-site-plugin

* [spotbugs] Fix incorrect lazy initialization in SystemReader

* [spotbugs] Don't use class from java.util.concurrent for locking

* [spotbugs] Fix potential NPE in OpenSshServerKeyDatabase

* [spotbugs] Fix potential NPE in FS.FileStoreAttributes#get

* [spotbugs] Silence CN_IMPLEMENTS_CLONE_BUT_NOT_CLONEABLE in jgit tests

* [spotbugs] Silence NP_BOOLEAN_RETURN_NULL in FakeUserAuthGSS#doAuth

* [spotbugs] Fix FileReftableStack#equals to check for null

* [spotbugs] FileReftableDatabase: extract lock to local variable

* [spotbugs] DfsReftableDatabase: extract lock to local variable

* [spotbugs] Silence NP_BOOLEAN_RETURN_NULL in IgnoreNode#checkIgnored

* [spotbugs] Silence warning DLS_DEAD_LOCAL_STORE in generated classes

* [spotbugs] Silence warning about unwritten field written by gson

* Merge branch 'stable-5.10'

* Revert "Remove unused API problem filters"

* Remove unused API problem filters

* [spotbugs] Fix potential NPE in FS#write

* [spotbugs] Fix potential NPE in PackFileSnapshotTest

* [spotbugs] Fix potential NPEs in FileReftableStackTest

* [spotbugs] Fix potential NPE in WorkingTreeIterator#isModified

* [spotbugs] Fix potential NPE in FileBasedConfigTest

* Update jgit-4.18 target platform to use release repository

* [spotbugs] Fix potential NPE in GcPruneNonReferencedTest

* [spotbugs] Fix potential NPE in FileRepository#convertToReftable

* [spotbugs] silence warnings for intended use of == to compare strings

* [spotbugs] silence short-circuit warning in PackOutputStream#write

* [spotbugs]  parse time using thread-safe DateTimeFormatter

* FetchProcess#execute: fix formatting

* [errorprone] FetchProcess: ensure exception isn't suppressed

* [errorprone] WalkFetchConnection: ensure exception isn't suppressed

* Remove unused API problem filters
* Remove reftree and ketch
* LsRemote: suppress warning for non-externalized string
* LooseObjects: fix formatting

* Split out loose object handling from ObjectDirectory

* Merge branch 'stable-5.9'

* Fix stamping to produce stable file timestamps

* PackDirectory: fix formatting
* Split out packfile handling from ObjectDirectory