TagCommand: propagate NO_CHANGE information

TagCommand: propagate NO_CHANGE information

Some clients may wish to allow NO_CHANGE lightweight tag updates
without setting the force flag. (For instance EGit does so.)
Command-line git does not allow this.

Propagate the RefUpdate result via the RefAlreadyExistsException.
That way a client has the possibility to catch it and check the
failure reason without having to parse the exception message, and
take appropriate action, like ignoring the exception on NO_CHANGE.

Change-Id: I60e7a15a3c309db4106cab87847a19b6d24866f6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>