convert: secret config option for disabling debugsvnlog
Subversion issues involving svn log such as d54467c1a198 can be tricky to
debug when it is run in an 'hg debugsvnlog' sub process. Debugging is simpler
when convert only uses one process.
With this change convert will invoke the svn log directly when setting
[convert]
svn.debugsvnlog = False
This is intentionally not documented.
convert: make subversion revsplit more stable when meeting revisions without @
revsplit would crash for instance if given a subversion string without @ ...
and that could somehow happen when playing around with convert.