largefiles: fix a traceback in lfconvert if a largefile is missing (issue3519) The largefile may be missing for various reasons, including that a remote repository was cloned without the --all-largefiles option. Therefore, it seems reasonable to attempt to download the missing files and failing that, abort and indicate the affected file and revision so the user can manually fix the problem.

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.

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().