Don't regenerate RCS.log file unless at least one of the component RCS

Don't regenerate RCS.log file unless at least one of the component RCS
files has changed.  This will make the common case more convenient,
although it won't detect adding or removing files from the set being
merged; in those cases the RCS.log should be deleted.  Also, change
the order of argument to RCS-DIRECTORY-LOG, default the output-file
argument, and change it to be interpreted relative to the directory
being merged.