Single-repo OpenJDK projects would move from Mercurial to Git version control under a JDK Enhancement Proposal (JEP) being reviewed by Mark Reinhold, chief architect of the Java platform group at Oracle.
A primary motivation behind the effort is reducing the size of version control metadata, which would preserve disk space and reduce clone size. The proposal notes that the .git directory of the jdk/jdk repo is approximately 300 MB with Git while the .hg directory is around 1.2 GB with Mercurial, depending on the version of Mercurial in use.