Add removal of -- token after parsing revspec in diff

When using src 1.29 as src diff 5-7 -- file.org I would receive the error src: -- is not registered. In the argument parsing it appears that after the revspec is pop'ed the '--' token remains and can cause this error. I added the two-line fix after that and confirmed that it fixes the error, and all the tests pass. Unfortunately I am not skilled enough to add a regression test to srctest.