branch: add a dry_run parameter to create_branch()

Add a dry_run parameter to create_branch() such that dry_run = 1 will
validate a new branch without trying to create it. This will be used in
`git branch --recurse-submodules` to ensure that the new branch can be
created in all submodules.

Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -859,7 +859,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 
 		create_branch(the_repository,
 			      argv[0], (argc == 2) ? argv[1] : head,
-			      force, 0, reflog, quiet, track);
+			      force, 0, reflog, quiet, track, 0);
 
 	} else
 		usage_with_options(builtin_branch_usage, options);