rev-parse: -- is sometimes a flag and sometimes an arg?

``` $ git rev-parse unknown-tag unknown-tag fatal: ambiguous argument 'unknown-tag': unknown revision or path not in the working tree. Use '--' to separate paths from revisions, like this: 'git <command> [<revision>...] -- [<file>...]' ```

OK

``` $ git rev-parse unknown-tag -- fatal: bad revision 'unknown-tag' ```

Much nicer error.  But:

``` $ git rev-parse HEAD -- 113a928239196d0d9f70671517ce917071ceecf6 -- ```

That's not very nice.  Why is "--" treated as an arg?

``` $ git rev-parse HEAD 113a928239196d0d9f70671517ce917071ceecf6 ```

Looking at rev-parse code it SEEMS to be intentional, but I can't comprehend why.

Tim