godoc/redirect: support loading hg->git change map from a file

This isn't exposed through the godoc binary, as it will only be
used by the Google-specific deployment of godoc.

Change-Id: Id5808f3adcb7eb36a7ccd6e4960ce3f01179fe51
Reviewed-on: https://go-review.googlesource.com/1567
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>