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

amend: do a bare kill of temporary changeset Before this changeset the temporary changeset created by amend was made a precursors on the amend result. This add unnecessary complexity to the obsolescence graph. This temporary commit will probably disappear in the future. It is an unwanted byproduct of amend that nobody cares about.