push: allow pushing new branches with --force-with-lease

If there is no upstream information for a branch, it is likely that it
is newly created and can safely be pushed under the normal fast-forward
rules.  Relax the --force-with-lease check so that we do not reject
these branches immediately but rather attempt to push them as new
branches, using the null SHA-1 as the expected value.

In fact, it is already possible to push new branches using the explicit
--force-with-lease=<branch>:<expect> syntax, so all we do here is make
this behaviour the default if no explicit "expect" value is specified.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.h

@@ -87,7 +87,6 @@ struct ref {
 		force:1,
 		forced_update:1,
 		expect_old_sha1:1,
-		expect_old_no_trackback:1,
 		deletion:1,
 		matched:1;