x/build: rewrite Hg hashes in issue tracker

Some old issues refer to mercurial hashes in their issue tracker. There is a mapping at https://golang.org/change/XXXXX and the git commits have this data too (as a golang-hg header in the commit), but if you don't know that it can be hard to track down.

Instead of people having to know this, one could rewrite all codesite URLs in the issue tracker to become googlesource.com URLs.

example: https://github.com/golang/go/issues/2516 mentions http://code.google.com/p/go/source/detail?r=4e38cad33649.