convert: avoid svn log retrieval process cleanup
With huge history (like kdelibs), the process termination suddenly consumes a
lot of memory (from 700M to 1.3G+). Since the job is done, clean termination is
not required, just exit.
*** failed to import extension badext from .../badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
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