Stefan's cvsweb commit made me realize that I forgot to committed this
stuff after testing it, since I had fixed that bug already. Oops!

• Added "?only_on_branch=RELENG_2_2" (e.g.) support and form to select
• Make diff requests refer to foo.diff so browsers don't tend to overwrite source files with diff's
• Get rid of content-transfer-encoding: x-gzip since it's not sufficiently standard
• Parse PR references in log messages
• Add side-by-side as a diff option (?f=s)
• Use &html_header() to get consistent look for error messages
• Make checkout and diff into functions for flexibility