shelve: move patch extension to a string constant We are using 'name + ".patch"' pattern throughout the shelve code to identify the existence of a shelve with a particular name. In two cases however we use 'name + ".hg"' instead. This commit makes 'patch' be used in all places and "emphasizes" it by moving 'patch' to live in a constant. Also, this allows to extract file name without extension like this: f[:-(1 + len(patchextension))] instead of: f[:-6] which is good IMO. This is a first patch from this initial "obsshelve" series. This series does not include tests, although locally I have all of test-shelve.t ported to test obs-shelve as well. I will send tests later as a separate series.