Merge branch 'km/branch-get-push-while-detached'

"git <cmd> @{push}" on a detached HEAD used to segfault; it has
been corrected to error out with a message.

* km/branch-get-push-while-detached:
  branch_get_push: do not segfault when HEAD is detached

t/t1514-rev-parse-push.sh

@@ -60,4 +60,10 @@ test_expect_success '@{push} with push refspecs' '
 	resolve topic@{push} refs/remotes/origin/magic/topic
 '
 
+test_expect_success 'resolving @{push} fails with a detached HEAD' '
+	git checkout HEAD^0 &&
+	test_when_finished "git checkout -" &&
+	test_must_fail git rev-parse @{push}
+'
+
 test_done