Add ability to use git-describe options to influence Refs

I see in #1172 that specific types of references were targeted.

In my case - working with Linux kernel trees - the default Ref: reported isn't helpful. In almost all cases I need to see the equivalent of git describe --contains and additionally need to filter the returned tags with e.g. --exclude 'next-*' or --match ...

Would it be possible to add a command-line option that passes through git-describe options to the underlying call ?

I've had a look at the source and in principle it looks feasible to:

1. collect an additional command-line option
2. append it to describe_argv[] in src/diff.c by factoring out the hard-coded "--tags" into a variable.

I'm not clear on how one would get from 1 to 2 without a lot of digging so hopeful this might be trivial for someone with intimate knowledge of the code.