evolve: add comptability to read old evolvestate files
When a user runs `hg evolve` with old evolve where we used to write evolvestate
the old ways i.e. without using cbor, and faces conflicts, the evolve is
interrupted. If the user updates evolve before completing the evolve, that can
result in traceback as the new evolve cannot read the old evolvestate file.
The previous patch added function to read old evolvestate file and this patch
uses that when we encounter the above mentioned case.