> I am thinking a scheme like: hg:<commit ref>:path/to/file

For the Python bug tracker we just support path/to/file and
path/to/file:xxx, where xxx is the line number.  We also support that
for tracebacks.  We have no way to select the cs id/branch, even if
there is an rfe for branch support somewhere.

What I tried to do was to write regexes that matched what people were
already using, rather than having people using some specific markup to
create links.