Merge branch 'rr/push-head'

"git push $there HEAD:branch" did not resolve HEAD early enough, so
it was easy to flip it around while push is still going on and push
out a branch that the user did not originally intended when the
command was started.

* rr/push-head:
  push: make push.default = current use resolved HEAD
  push: fail early with detached HEAD and current
  push: factor out the detached HEAD error message

builtin/push.c

@@ -113,17 +113,19 @@ static NORETURN int die_push_simple(struct branch *branch, struct remote *remote
 	    remote->name, branch->name, advice_maybe);
 }
 
+static const char message_detached_head_die[] =
+	N_("You are not currently on a branch.\n"
+	   "To push the history leading to the current (detached HEAD)\n"
+	   "state now, use\n"
+	   "\n"
+	   "    git push %s HEAD:<name-of-remote-branch>\n");
+
 static void setup_push_upstream(struct remote *remote, int simple)
 {
 	struct strbuf refspec = STRBUF_INIT;
 	struct branch *branch = branch_get(NULL);
 	if (!branch)
-		die(_("You are not currently on a branch.\n"
-		    "To push the history leading to the current (detached HEAD)\n"
-		    "state now, use\n"
-		    "\n"
-		    "    git push %s HEAD:<name-of-remote-branch>\n"),
-		    remote->name);
+		die(_(message_detached_head_die), remote->name);
 	if (!branch->merge_nr || !branch->merge || !branch->remote_name)
 		die(_("The current branch %s has no upstream branch.\n"
 		    "To push the current branch and set the remote as upstream, use\n"
@@ -173,6 +175,8 @@ static void warn_unspecified_push_default_configuration(void)
 
 static void setup_default_push_refspecs(struct remote *remote)
 {
+	struct branch *branch;
+
 	switch (push_default) {
 	default:
 	case PUSH_DEFAULT_UNSPECIFIED:
@@ -192,7 +196,10 @@ static void setup_default_push_refspecs(struct remote *remote)
 		break;
 
 	case PUSH_DEFAULT_CURRENT:
-		add_refspec("HEAD");
+		branch = branch_get(NULL);
+		if (!branch)
+			die(_(message_detached_head_die), remote->name);
+		add_refspec(branch->name);
 		break;
 
 	case PUSH_DEFAULT_NOTHING: