bookmarks: further flatten code This hopefully clarifies the behavior when no NAME is passed, by separating the branches for listing bookmarks vs. deactivating the current bookmark.

scmutil: add bad character checking to checknewlabel This factors out the checks from tags and bookmarks, and newly applies the same prohibitions to branches. checknewlabel takes a new parameter, kind, indicating the kind of label being checked. Test coverage is added for all three types of labels.

tag: disallow '\0' in tag names This brings the list of disallowed characters in line with bookmarks.

dirstate: use scmutil.checknewlabel to check new branch name

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.