shelve: move possible shelve file extensions to a single place This and a couple of following patches are a preparation to implementing obsolescense-enabled shelve which was discussed on a Sprint. If this refactoring is not done, shelve is going to look even more hackish than now. This particular commit introduces a slight behavior change. Previously, if only .hg/shelve/name.patch file exists, but .hg/name.hg does not, 'hg shelve -d name' would fail saying "shelve not found". Now deletion will only fail if .patch file does not exist (since .patch is used as an indicator of an existing shelve). Other shelve files being absent are skipped silently to accommodate for future introduction of obs-based shelve, which will mean that for some shelves .hg and .patch files exist, while for others .hg and .oshelve.