pg_backup_and_purge: Fix buggy recovery status parsing.
pg_backup_and_purge: Fix buggy recovery status parsing.

This was converted to Python 3 incorrectly, in a way that actually
completely broke the script (the .decode() that this adds is critical,
since 'f' != b'f').

We fix this, and also add an assert that makes the parsing code
safer against future refactors.

tommyip/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,showell/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,kou/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,rishig/zulip,kou/zulip,showell/zulip,tommyip/zulip,tommyip/zulip,rht/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,jackrzhang/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,andersk/zulip,shubhamdhama/zulip,dhcrzf/zulip,jackrzhang/zulip,shubhamdhama/zulip,eeshangarg/zulip,andersk/zulip,jackrzhang/zulip,punchagan/zulip,hackerkid/zulip,tommyip/zulip,tommyip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,dhcrzf/zulip,shubhamdhama/zulip,tommyip/zulip,rishig/zulip,rht/zulip,punchagan/zulip,timabbott/zulip,brainwane/zulip,dhcrzf/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,tommyip/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,shubhamdhama/zulip,brainwane/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,jackrzhang/zulip,synicalsyntax/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,rishig/zulip,dhcrzf/zulip,punchagan/zulip,jackrzhang/zulip,rishig/zulip,synicalsyntax/zulip,rishig/zulip,synicalsyntax/zulip,timabbott/zulip,jackrzhang/zulip,andersk/zulip,timabbott/zulip,hackerkid/zulip,shubhamdhama/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,hackerkid/zulip,timabbott/zulip,andersk/zulip,showell/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,dhcrzf/zulip,rishig/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,dhcrzf/zulip,hackerkid/zulip,brainwane/zulip