docs: update credtis, point to the repository log

The docs/CREDITS file dates back to the SVN repository.
Now that we use Mercurial, the repository stores appropriate
authorship for each commit. Say so in the CREDITS.

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =