Support atomic push in JGit client

Support atomic push in JGit client

This should mirror the behavior of `git push --atomic` where the
client asks the server to apply all-or-nothing. Some JGit servers
already support this based on a custom DFS backend. InMemoryRepository
is extended to support atomic push for unit testing purposes.

Local disk server side support inside of JGit is a more complex animal
due to the excessive amount of file locking required to protect every
reference as a loose reference.

Change-Id: I15083fbe48447678e034afeffb4639572a32f50c