GitHub のミラー のほうが高速なので、こちらから入手します。

bionic64:~/src$ git clone https://github.com/openjdk/jdk.git
Cloning into 'jdk'...
remote: Enumerating objects: 441, done.
remote: Counting objects: 100% (441/441), done.
remote: Compressing objects: 100% (215/215), done.
remote: Total 924430 (delta 212), reused 377 (delta 206), pack-reused 923989
Receiving objects: 100% (924430/924430), 293.38 MiB | 4.15 MiB/s, done.
Resolving deltas: 100% (701186/701186), done.
Checking out files: 100% (67000/67000), done.

OpenJDK 11 の最新タグを checkout します。

bionic64:~/src$ cd jdk/
bionic64:~/src/jdk$ git tag|grep jdk-11
jdk-11+0
jdk-11+1
jdk-11+10
jdk-11+11
jdk-11+12
jdk-11+13
jdk-11+14
jdk-11+15
jdk-11+16
jdk-11+17
jdk-11+18
jdk-11+19
jdk-11+2
jdk-11+20
jdk-11+21
jdk-11+22
jdk-11+23
jdk-11+24
jdk-11+25
jdk-11+26
jdk-11+27
jdk-11+28
jdk-11+3
jdk-11+4
jdk-11+5
jdk-11+6
jdk-11+7
jdk-11+8
jdk-11+9
bionic64:~/src/jdk$ git checkout jdk-11+28 -b jdk11
Checking out files: 100% (14821/14821), done.
Switched to a new branch 'jdk11'