Merge branch 'db/maint-checkout-b' into maint

* db/maint-checkout-b:
Check early that a new branch is new and valid

diff --combined builtin-checkout.c
index c4fc2b2c562725018789dead3b86ccfbc4e2925f,1ee23468ff27de8f8d35c15ce6b10ac11995c0f9..1deda927cdfcbcdcb751aca3338ac77ef0aad836
--- 1/builtin-checkout.c
--- 2/builtin-checkout.c
+++ b/builtin-checkout.c
@@@ -328,7 -328,7 +328,7 @@@ static int merge_working_tree(struct ch
            commit_locked_index(lock_file))
                die("unable to write new index file");
  
 -      if (!opts->force)
 +      if (!opts->force && !opts->quiet)
                show_local_changes(&new->commit->object);
  
        return 0;
@@@ -565,6 -565,18 +565,18 @@@ no_reference
                return checkout_paths(source_tree, pathspec);
        }
  
+       if (opts.new_branch) {
+               struct strbuf buf;
+               strbuf_init(&buf, 0);
+               strbuf_addstr(&buf, "refs/heads/");
+               strbuf_addstr(&buf, opts.new_branch);
+               if (!get_sha1(buf.buf, rev))
+                       die("git checkout: branch %s already exists", opts.new_branch);
+               if (check_ref_format(buf.buf))
+                       die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
+               strbuf_release(&buf);
+       }
+ 
        if (new.name && !new.commit) {
                die("Cannot switch branch to a non-commit.");
        }