Non-Fast-Forward Ref-Updates: Omit isMergedInto() calls

Non-Fast-Forward Ref-Updates: Omit isMergedInto() calls

When the caller specifies to JGit in advance that a ref-update is a
non-fast-forward update, and that those are permitted, we should never
need to call the potentially expensive isMergedInto() check. Re-checking
that the older commit is /not/ reachable from the newer is superfluous.

http://dev.eclipse.org/mhonarc/lists/jgit-dev/msg02258.html

Change-Id: I4bbf593de4dcea6b6f082881c1a33cb3a6a7fb89
Signed-off-by: Roberto Tyley <roberto.tyley@gmail.com>