Using rcs2log on a cvs branch

[Martin Stjernholm]

If I run rcs2log normally, it finds the date of the last entry in the
changelog file and only include changes from that date, which is nice.
However, if I want to run it on a branch I have to give an extra
argument to cvs log since it always defaults to the main branch:

    rcs2log -r -rMyBranch

But with this rcs2log no longer looks at the last entry in the
changelog file, and so I get the whole branch. Some investigation of
the source reveals that it avoids adding a date filter argument if any
-r option is given, something that isn't mentioned in the brief help
text.

Suggestion 1: rcs2log should be more careful about dropping its
generated date filter argument. It needs to be done only if there's
some -r argument that passes on a -d argument to the log command.

Suggestion 2: When rcs2log is used in a CVS tree where there is a
CVS/Tag file containing a branch tag, it could automatically generate
a -rTheTag argument to the cvs log command. That since it's reasonable
to assume that the log entries should pertain to the branch that is
checked out where the command is run, and not to the main branch.
(Ideally the cvs log command itself should do this by default.)

The attached patch implements both suggestions.

rcs2log.patch
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