[Integrated] RFR: Add git-sync command

[duke]

Changeset: 6ba889e2
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2019-09-24 12:13:22 +0000
URL:       https://git.openjdk.java.net/skara/commit/6ba889e2

Add git-sync command

Reviewed-by: rwestberg, jvernee

! cli/build.gradle
! cli/src/main/java/org/openjdk/skara/cli/GitFork.java
! cli/src/main/java/org/openjdk/skara/cli/GitPr.java
! cli/src/main/java/org/openjdk/skara/cli/GitSkara.java
+ cli/src/main/java/org/openjdk/skara/cli/GitSync.java
+ cli/src/main/java/org/openjdk/skara/cli/Remote.java
! jcheck/src/test/java/org/openjdk/skara/jcheck/TestRepository.java
! skara.gitconfig
! vcs/src/main/java/org/openjdk/skara/vcs/ReadOnlyRepository.java
+ vcs/src/main/java/org/openjdk/skara/vcs/Reference.java
! vcs/src/main/java/org/openjdk/skara/vcs/git/GitRepository.java
! vcs/src/main/java/org/openjdk/skara/vcs/hg/HgRepository.java
! vcs/src/main/resources/ext.py
! vcs/src/test/java/org/openjdk/skara/vcs/RepositoryTests.java