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parsers: a C implementation of the new ancestors algorithm

The performance of both the old and new Python ancestor algorithms
depends on the number of revs they need to traverse.  Although the
new algorithm performs far better than the old when revs are
numerically and topologically close, both algorithms become slow
under other circumstances, taking up to 1.8 seconds to give answers
in a Linux kernel repo.

This C implementation of the new algorithm is a fairly straightforward
transliteration.  The only corner case of interest is that it raises
an OverflowError if the number of GCA candidates found during the
first pass is greater than 24, to avoid the dual perils of fixnum
overflow and trying to allocate too much memory.  (If this exception
is raised, the Python implementation is used instead.)

Performance numbers are good: in a Linux kernel repo, time for "hg
debugancestors" on two distant revs (24bf01de7537 and c2a8808f5943)
is as follows:

  Old Python: 0.36 sec
  New Python: 0.42 sec
  New C: 0.02 sec

For a case where the new algorithm should perform well:

  Old Python: 1.84 sec
  New Python: 0.07 sec
  New C: measures as zero when using --time

(This commit includes a paranoid cross-check to ensure that the
Python and C implementations give identical answers. The above
performance numbers were measured with that check disabled.)

#!/usr/bin/env python
#
# An example hgweb CGI script, edit as necessary
# See also http://mercurial.selenic.com/wiki/PublishingRepositories

# Path to repo or hgweb config to serve (see 'hg help hgweb')
config = "/path/to/repo/or/config"

# Uncomment and adjust if Mercurial is not installed system-wide
# (consult "installed modules" path from 'hg debuginstall'):
#import sys; sys.path.insert(0, "/path/to/python/lib")

# Uncomment to send python tracebacks to the browser if an error occurs:
#import cgitb; cgitb.enable()

from mercurial import demandimport; demandimport.enable()
from mercurial.hgweb import hgweb, wsgicgi
application = hgweb(config)
wsgicgi.launch(application)