mdiff: replace wscleanup() regexps with C loops
On my system it reduces:
hg annotate -w mercurial/commands.py
from 36s to less than 8s, to be compared with 6.3s when run without whitespace
options.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html