bookmarks: use scmutil.checknewlabel Validation is pulled up into the commands module to avoid an import cycle.

scmutil: add function to validate new branch, tag, and bookmark names For now the new function only checks to make sure the new label name isn't a reserved name ('tip', '.', or 'null'). Eventually more of the checks will be unified between the different types of labels. The `tag` command is trivially updated to use it. Updating branches and bookmarks to use it is slightly more invasive and thus reserved for later patches.

bookmarks: disallow bookmarks named 'tip', '.', or 'null' This makes bookmarks reject the same reserved names as tags and branches.

bookmarks: abort directly on invalid name This consolidates the abort message for a bookmark name containing invalid characters into one place, and renames the valid() method to checkvalid() to reflect the fact that it's no longer a predicate.

bookmarks: remove redundant check for newline New bookmarks are already checked for illegal characters (':', '\0', '\n', and '\r') in bookmarks.valid().

test-tag: test that all reserved names are rejected