tests/sitecustomize.py

transaction: fix file descriptor leak for journal.backupfiles

The journal.backupfiles descriptor wasn't being closed. This resulted in
hgsubversion test runs having a bagillion descriptors open, which crashed on
platforms with low open file limits (like OSX).

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