Add infrastructure for links to Git commits.

• Add a <githash> tag which accepts a Git hash. Leave <revnumber> as is for svn revisions. In theory we could add a new attribute to <revnumber> to say which VCS to use, but this would require tagging either all SVN revisions or all Git hashes explicitly which would be invasive. A new tag going forward seems simpler.

• Revert svnweb.link back to generating svnweb links. This fixes all the explicit <revnumber>'s referring to SVN src commits in existing documentation.

• Add a cgit.link which links to a Git commit in cgit. githash generates cgit.link's when asked to generate a href.

Reviewed by: gjb, ygy
Differential Revision: https://reviews.freebsd.org/D27703