bookmark: don't allow integers as bookmark/branch/tag names
Bookmarks/branches/tags shouldn't be allowed to be integers because that
overlaps with revision numbers. Right now if a user created one they can't
use it anyway because the revision numbers take precedence.
The check only happens when creating a new bookmark/etc from a command so it
shouldn't affect existing bookmarks/branches/tags or importing branches from
git.
This fix was prompted by us having a user create a bookmark named "404" then
accidentally checkout a very old version of our repository.