py3: don't return the revid as unicode in hgext/convert/subversion.py
I tried digging why u'' was added in first place, and I was unable to found
something relevant. This might be because some API's takes unicodes, I am not
sure.
Differential Revision: https://phab.mercurial-scm.org/D4454
py3: make sure we pass str in os.sysconf in hgext/convert/common.py
# skip-blame because just r'' prefix
Differential Revision: https://phab.mercurial-scm.org/D4453