Abbreviate SHA-1 in output to client

getId().toString() returns an ugly internal JGit string listing the
current revision flags attached to the RevCommit.  This is not
interesting or relevant to the end-user of Gerrit.

Instead abbreviate the SHA-1 to be 8 hex digits and print that as a
clean string.  For most Git repositories this will be uniquely
identified.  It is shorter to use in commands like `git show` when the
end-user wants to inspect a commit that was rejected by the CLA
plugin.