archive: store number of changes since latest tag as well This is different from latesttagdistance in that while latesttagdistance is defined to be the length of the longest path to the latest tag, changessincelatesttag is the number of changes contained in @ that aren't contained in the latest tag. So, if 't' is the latest tag in the repository below: t | v --o--o----o \ \ ..o..o..@ then latesttagdistance is 2, but changessincelatesttag is 4. Note that changessincelatesttag is always greater than or equal to the latesttagdistance -- that's because changessincelatesttag counts all the changes in the longest path since the latest tag, and possibly others. This is an important fact that we'll take advantage of in upcoming patches.