revlog: switch to a C version of headrevs
The C implementation is more than 100 times faster than the Python
version (which is still available as a fallback).
In a repo with 330,000 revs and a stale .hg/cache/tags file, this
patch improves the performance of "hg tip" from 2.2 to 1.6 seconds.
try:
import coverage
getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
pass