Work also when the working directory (with e.g. coreutils sources)

Work also when the working directory (with e.g. coreutils sources)
is version controlled with git, rather than CVS.
* bootstrap (CVS_only_file): Test for the existence of README-cvs,
rather than CVS.
In messages and comments, say e.g., "checked-out sources",
rather than "CVS sources".
(version_controlled_file): New function.  Work for git as well as
for CVS.  Don't use grep's -q option.
(slurp): Call it here, in place of CVS-specific code.