branchcache: improve speed relative to the amount of heads
Updating the branch cache is quadratic to the amount of heads in the
repository. One consequence of this was that cloning a pathological
repository with 10,000 heads (and nothing else) took hours of CPU
time.
This patch makes one of the inner loop much faster, by removing a
changectx instantiation, and removes another entirely in cases where
there are no candidate branch heads which descend from other branch
heads.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.