tests/test-context.py.out

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.

workingfilectx.date = (1000, 0)
ASCII   : Gr?ezi!
Latin-1 : Grüezi!
UTF-8   : Grüezi!