Use JGit's recursive merge

This is now the default in JGit, and matches the behavior of
native git clients.

Affects repos using content merges (merge always, merge if
necessary), which is about 95% of our repositories.

Change-Id: I445460a6e05f4bae5308a25b07bc9fdcbe44cdc2
Merge "Use JGit's recursive merge" into production