[push, lfs] Tell the pre-push hook whether the push is a dry run

[push, lfs] Tell the pre-push hook whether the push is a dry run

This is a feature that does not exist in C git: an external pre-push
hook doesn't know whether the push is run as a dry run. But for
internal hooks written in Java it is entirely possible to give a hook
this information.

In JGit with its internal LFS implementation, this enables us to not
perform LFS uploads in a dry run. This is kind of important because
EGit frequently does a dry-run before doing the actual push to give the
user a way to review what would be pushed before it actually happens.

Doing an LFS upload of potentially huge files during a dry-run is
wasteful, makes the dry run not actually a dry run, and leads to
uploading the same file twice if followed by a real push.

Use the information in the LfsPrePushHook to only do the initial call
to the LFS server, but then skipping the actual upload if the push is
a dry run. That way, a failure to contact the LFS server leads to an
error in the dry run, as it should.

Bug: 546567
Change-Id: I155430f27c4979d91096ba72fd95c3775dd3f28b
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>