tests/sitecustomize.py

annotate: simplify annotate parent function

The annotate algorithm used a custom parents() function to try to reuse
filectx and filelogs. I simplified it a bit to rely more heavily on the
self.parents() which makes it work well with alternative filectx
implementations. I tested performance on a file with 5000+ revisions
but no renames, and on a file with 500 revisions repeating a series of
4 edits+renames and saw zero performance hit.  In fact, it was reliably a
couple milliseconds faster now.

Added the perfannotate command to contrib/perf.py for future use.

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