Add reference repo for git clone openj9

[llxia]

related: runtimes/backlog/issues/815

Signed-off-by: lanxia lan_xia@ca.ibm.com

[llxia]

• the cache does not exist (warning message): https://ci.adoptopenjdk.net/view/Test_grinder/job/Grinder/5304/

[2022-07-27T15:07:07.944Z] git clone --depth 1 --reference-if-able /home/jenkins/openjdk_cache -b master https://github.com/eclipse/openj9.git
[2022-07-27T15:07:07.944Z] Cloning into 'openj9'...
[2022-07-27T15:07:07.944Z] info: Could not add alternate for '/home/jenkins/openjdk_cache': reference repository '/home/jenkins/openjdk_cache' is not a local repository.
[2022-07-27T15:07:16.833Z] Updating files:  96% (9714/10083)
Updating files:  97% (9781/10083)
Updating files:  98% (9882/10083)
Updating files:  99% (9983/10083)
Updating files: 100% (10083/10083)
Updating files: 100% (10083/10083), done.
[2022-07-27T15:07:16.833Z] check OpenJ9 Repo sha

• cache exists and is up to date: Grinder/26236

[2022-07-27T15:20:03.267Z] git clone --depth 1 --reference-if-able /home/jenkins/openjdk_cache -b master https://github.com/eclipse-openj9/openj9.git
[2022-07-27T15:20:03.267Z] Cloning into 'openj9'...
[2022-07-27T15:20:09.528Z] Checking out files:  43% (4393/10083)   
Checking out files:  44% (4437/10083)   
Checking out files:  45% (4538/10083)   
...
Checking out files: 100% (10083/10083), done.
[2022-07-27T15:20:09.528Z] check OpenJ9 Repo sha

[llxia]

Further simplification will be done in #3828

[renfeiw]

LGTM

[karianna]

might be worth adding a comment as to why this is being added.

[llxia]

might be worth adding a comment as to why this is being added.

This is to improve the git clone efficiency.

Using an already existing repository as an alternate will require fewer objects to be copied from the repository being cloned, reducing network and local storage costs. When using the --reference-if-able, a non existing directory is skipped with a warning instead of aborting the clone.

https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---reference-if-ableltrepositorygt

[AdamBrousseau]

Came back to this looking for this info so I'll add it here.

The --reference-if-able needs git >= 2.11.4