templatekw: add a {negrev} keyword Revision numbers are getting much maligned for two reasons: they are too long in large repos and users get confused by their local-only nature. It just occurred to me that negative revision numbers avoid both of those problems. Since negative revision numbers change whenever the repo changes, it's much more obvious that they are a local-only convenience. Additionally, for the recent commits that we usually care about the most, negative revision numbers are always near zero. This commit adds a negrev templatekw to more easily expose negative revision numbers. It's not easy to reliably produce this output with existing keywords due to hidden commits while at the same time ensuring good performance.