templater: add count template filter, plus tests Previously there was no way of telling how much children or bookmarks or tags a certain changeset has in a template. It was possible to tell if a changeset has either 0 or not 0 bookmarks, but not to tell if it has 1 or 2 of them, for example. This filter, simply named count, makes it possible to count the number of items in a list or the length of a string (or, anything that python's len can count). E.g.: {children|count}, {bookmarks|count}, {file_adds|count}. Testing the filter on node hash and shortened node hash is chosen because they both have defined length. As for lists of strings - children, tags and file_adds are used, because they provide some variety and also prove that what's counted is the number of string items in the list, and not the list stringified (they are lists of non-empty, multi-character strings). Additionally, revset template function is used for testing the filter, since the combination is very flexible and will possibly be used together a lot. (The previous version of this patch had an incorrect email subject and was apparently lost - patchwork says the patch has been accepted, but it's not so. The changes between that and this patch are minimal: now the filter does not disturb the alphabetical order of function definitions and dict keys.)