gitlog-to-changelog: pass all command-line arguments to git-log

gitlog-to-changelog: pass all command-line arguments to git-log

When producing a ChangeLog, it is sometimes convenient
to filter the commits in various ways.  gitlog-to-changelog
only allows --since to specify a start date, but git-log
itself supports many other filtering mechanisms.  At the
moment, I want to filter by branch name.  Rather than adding
a --branch option to gitlog-to-changelog, it seems more
flexible to simply pass all options directly to git-log
and let git do the work.  Notice that this effectively
makes --since a redundant option for gitlog-to-changelog,
but removing it would require current usage to change since
calls would then require an additional '--'.