tests/sitecustomize.py

convert: check for failed svn import in debugsvnlog and abort cleanly

'hg debugsvnlog' failed with a crash when using the uninitialized transport in
get_log_child if the import of the svn libraries had failed.

'convert' should never get as far as launching 'hg debugsvnlog' if the svn
libraries are missing, but by launching a subprocess there is risk that the
environment is mangled so the second import fails.

It is in principle also possible to launch the command manually.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass