shelve: make shelvestate use simplekeyvaluefile Currently shelvestate uses line ordering to differentiate fields. This makes it hard for extensions to wrap shelve, since if two alternative versions of code add a new line, correct merging is going to be problematic. simplekeyvaluefile was introduced fot this purpose specifically. After this patch: - shelve will always write a simplekeyvaluefile - unshelve will check the first line of the file for a version, and if the version is 1, will read it in a position-based way, if the version is 2, will read it in a key-value way As discussed with Yuya previously, this will be able to handle old-style shelvedstate files, but old Mercurial versions will fail on the attempt to read shelvedstate file of version 2 with a self-explanatory message: 'abort: this version of shelve is incompatible with the version used in this repo'