Fix Py3 compat and logic/syntax in handling version.txt

Fix Py3 compat and logic/syntax in handling version.txt

The first fix addresses the problem that readlines() gives back a
list of strings, which do not have the `.decode()` method that bytes do.
But `_parseSession()` is used both for the bytes output by running
`repo-info.sh` and for the strings output by ingesting `version.txt`.
So we still need to handle the bytes case.

The second fix addresses syntax and logic problems. If a string is
not empty, `if self.hash` will check that it's non-empty (I don't think
`is not empty` actually works -- it has given me a NameError). And the
string comparison should be done by value equality, not reference
equality.