shelve: some docstring cleanups

dirstate.status: return explicit unknown files even when not asked dirstate.walk will return unknown files that were explicitly requested, even if listunknown is false. There's no point in dropping these files on the floor in dirstate.status. This has no effect on any current callers, because all of them assume the unknown list is empty and ignore it. Future callers may find it useful, though.

shelve: add minimal documentation to all functions There is a lot of functions in this extension. We had a small documentation help people getting started.

shelve: use the class constant in the clear method This class attribut is used everywhere else. Not using it in clear will get us in trouble sooner or later.

hgweb: fix unstoppable loading of graph when no more results When a user reaches the end of repo history with graph view, it (unsuccessfully) tried to load more entries. This patch disables further loading attempts.

hgweb: remove now unnecessary explicit header() and footer() They became unnecessary after allowing custom-named entries in templates.

hgweb: replace 'shortlog' with 'changelog' in raw changelog template

shelve: drop pickle usage Pickle was used to the `shelvedstate` file. However the content of the file is very simple and we can handle serialisation ourself. Not using pickle is a net win. Note incrementing the format version as no releases have been done so far.

store: use "vfs.exists()" instead of "os.path.exists()"

context: use "vfs.lstat()" to examine target path instead of "os.path.*" This patch gets stat object of target path by "vfs.lstat()", and examines stat object to know the type of it. This follows the way in "workingctx.add()". This should be cheaper than original implementation invoking "lexists()", "isfile()" and "islink()".