util: eliminate the possibility of returning None from `versiontuple()`
This fixes the following error flagged by pytype:
File "/mnt/c/Users/Matt/hg/mercurial/extensions.py", line 228, in load: unsupported operand types for > [unsupported-operands]
Primitive types 'Optional[tuple]' and 'curver: Optional[tuple]' are not comparable.
Differential Revision: https://phab.mercurial-scm.org/D11473
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..